尝试一下

 Rectangle bounds = Screen.GetBounds(Point.Empty);
 using (Bitmap bitmap = new Bitmap(bounds.Width, bounds.Height))
 {
    using (Graphics g = Graphics.FromImage(bitmap))
    {
       g.CopyFromScreen(Point.Empty, Point.Empty, bounds.Size);
    }
    bitmap.Save("c://My_Img.jpg", ImageFormat.Jpeg);
 }