我自己找到了解决方案.

I find the solution by myself.

事实上，使用 OpenXml 在 docx 中添加页眉或页脚涉及:

In fact, adding a header or a footer in a docx with OpenXml involves that :

文档必须存在.

在添加页眉或页脚之前保存和关闭文档不起作用.

Saving and closing document before adding a header or a footer does not work.

WordprocessingDocument document = WordprocessingDocument.Open(path, true);
<operations on docx>
document.Save();
document.Close();
ChangeHeader(document);

您必须再次打开 docx 才能创建文档的新实例.

You have to re-open a second time your docx to create a new instanciation of the document.

正确的解决方案是(使用 James Wood 解决方案):

Correct solution will be (with James Wood solution):

WordprocessingDocument document = WordprocessingDocument.Open(path, true);
<operations on docx>
document.Save();
document.Close();
ChangeHeader(path);

    public static void ChangeHeader(String documentPath)
    {
        // Replace header in target document with header of source document.
        using (WordprocessingDocument document = WordprocessingDocument.Open(documentPath, true))
        {
            // Get the main document part
            MainDocumentPart mainDocumentPart = document.MainDocumentPart;

            // Delete the existing header and footer parts
            mainDocumentPart.DeleteParts(mainDocumentPart.HeaderParts);
            mainDocumentPart.DeleteParts(mainDocumentPart.FooterParts);

            // Create a new header and footer part
            HeaderPart headerPart = mainDocumentPart.AddNewPart<HeaderPart>();
            FooterPart footerPart = mainDocumentPart.AddNewPart<FooterPart>();

            // Get Id of the headerPart and footer parts
            string headerPartId = mainDocumentPart.GetIdOfPart(headerPart);
            string footerPartId = mainDocumentPart.GetIdOfPart(footerPart);

            GenerateHeaderPartContent(headerPart);

            GenerateFooterPartContent(footerPart);

            // Get SectionProperties and Replace HeaderReference and FooterRefernce with new Id
            IEnumerable<SectionProperties> sections = mainDocumentPart.Document.Body.Elements<SectionProperties>();

            foreach (var section in sections)
            {
                // Delete existing references to headers and footers
                section.RemoveAllChildren<HeaderReference>();
                section.RemoveAllChildren<FooterReference>();

                // Create the new header and footer reference node
                section.PrependChild<HeaderReference>(new HeaderReference() { Id = headerPartId });
                section.PrependChild<FooterReference>(new FooterReference() { Id = footerPartId });
            }
        }
    }


public static void GenerateHeaderPartContent(HeaderPart part)
        {
            Header header1 = new Header() { MCAttributes = new MarkupCompatibilityAttributes() { Ignorable = "w14 wp14" } };
            header1.AddNamespaceDeclaration("wpc", "http://schemas.microsoft.com/office/word/2010/wordprocessingCanvas");
            header1.AddNamespaceDeclaration("mc", "http://schemas.openxmlformats.org/markup-compatibility/2006");
            header1.AddNamespaceDeclaration("o", "urn:schemas-microsoft-com:office:office");
            header1.AddNamespaceDeclaration("r", "http://schemas.openxmlformats.org/officeDocument/2006/relationships");
            header1.AddNamespaceDeclaration("m", "http://schemas.openxmlformats.org/officeDocument/2006/math");
            header1.AddNamespaceDeclaration("v", "urn:schemas-microsoft-com:vml");
            header1.AddNamespaceDeclaration("wp14", "http://schemas.microsoft.com/office/word/2010/wordprocessingDrawing");
            header1.AddNamespaceDeclaration("wp", "http://schemas.openxmlformats.org/drawingml/2006/wordprocessingDrawing");
            header1.AddNamespaceDeclaration("w10", "urn:schemas-microsoft-com:office:word");
            header1.AddNamespaceDeclaration("w", "http://schemas.openxmlformats.org/wordprocessingml/2006/main");
            header1.AddNamespaceDeclaration("w14", "http://schemas.microsoft.com/office/word/2010/wordml");
            header1.AddNamespaceDeclaration("wpg", "http://schemas.microsoft.com/office/word/2010/wordprocessingGroup");
            header1.AddNamespaceDeclaration("wpi", "http://schemas.microsoft.com/office/word/2010/wordprocessingInk");
            header1.AddNamespaceDeclaration("wne", "http://schemas.microsoft.com/office/word/2006/wordml");
            header1.AddNamespaceDeclaration("wps", "http://schemas.microsoft.com/office/word/2010/wordprocessingShape");

            Paragraph paragraph1 = new Paragraph() { RsidParagraphAddition = "00164C17", RsidRunAdditionDefault = "00164C17" };

            ParagraphProperties paragraphProperties1 = new ParagraphProperties();
            ParagraphStyleId paragraphStyleId1 = new ParagraphStyleId() { Val = "Header" };

            paragraphProperties1.Append(paragraphStyleId1);

            Run run1 = new Run();
            Text text1 = new Text();
            text1.Text = "Header";

            run1.Append(text1);

            paragraph1.Append(paragraphProperties1);
            paragraph1.Append(run1);

            header1.Append(paragraph1);

            part.Header = header1;
        }

        public static void GenerateFooterPartContent(FooterPart part)
        {
            Footer footer1 = new Footer() { MCAttributes = new MarkupCompatibilityAttributes() { Ignorable = "w14 wp14" } };
            footer1.AddNamespaceDeclaration("wpc", "http://schemas.microsoft.com/office/word/2010/wordprocessingCanvas");
            footer1.AddNamespaceDeclaration("mc", "http://schemas.openxmlformats.org/markup-compatibility/2006");
            footer1.AddNamespaceDeclaration("o", "urn:schemas-microsoft-com:office:office");
            footer1.AddNamespaceDeclaration("r", "http://schemas.openxmlformats.org/officeDocument/2006/relationships");
            footer1.AddNamespaceDeclaration("m", "http://schemas.openxmlformats.org/officeDocument/2006/math");
            footer1.AddNamespaceDeclaration("v", "urn:schemas-microsoft-com:vml");
            footer1.AddNamespaceDeclaration("wp14", "http://schemas.microsoft.com/office/word/2010/wordprocessingDrawing");
            footer1.AddNamespaceDeclaration("wp", "http://schemas.openxmlformats.org/drawingml/2006/wordprocessingDrawing");
            footer1.AddNamespaceDeclaration("w10", "urn:schemas-microsoft-com:office:word");
            footer1.AddNamespaceDeclaration("w", "http://schemas.openxmlformats.org/wordprocessingml/2006/main");
            footer1.AddNamespaceDeclaration("w14", "http://schemas.microsoft.com/office/word/2010/wordml");
            footer1.AddNamespaceDeclaration("wpg", "http://schemas.microsoft.com/office/word/2010/wordprocessingGroup");
            footer1.AddNamespaceDeclaration("wpi", "http://schemas.microsoft.com/office/word/2010/wordprocessingInk");
            footer1.AddNamespaceDeclaration("wne", "http://schemas.microsoft.com/office/word/2006/wordml");
            footer1.AddNamespaceDeclaration("wps", "http://schemas.microsoft.com/office/word/2010/wordprocessingShape");

            Paragraph paragraph1 = new Paragraph() { RsidParagraphAddition = "00164C17", RsidRunAdditionDefault = "00164C17" };

            ParagraphProperties paragraphProperties1 = new ParagraphProperties();
            ParagraphStyleId paragraphStyleId1 = new ParagraphStyleId() { Val = "Footer" };

            paragraphProperties1.Append(paragraphStyleId1);

            Run run1 = new Run();
            Text text1 = new Text();
            text1.Text = "Footer";

            run1.Append(text1);

            paragraph1.Append(paragraphProperties1);
            paragraph1.Append(run1);

            footer1.Append(paragraph1);

            part.Footer = footer1;
        }