瑞安，

我尝试制作一个示例进行测试，但效果很好，请注意，您可以设置ParentForm的 IsMdiContainer 属性设置为true，其WindowsState属性设置为true Maximized.

I try to do a sample to test it, and it works fine, please notice that you could set ParentForm's IsMdiContainer property to true, and its WindowsState property to Maximized.

并参考下面的代码.

Protected Sub MDIChildNew_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MenuItem2.Click  
   Dim NewMDIChild As New Form2()  
   'Set the Parent Form of the Child window.  
   NewMDIChild.MdiParent = Me  
   'Display the new form.  
   NewMDIChild.Show()  
End Sub  

有关更多详细信息，请参考 如何创建MDI子表单.

希望对您有帮助.

About more detailed info, please refer to How to create MDI Child Forms.

Hope it is helpful to you.

***的问候，

樱桃