As you can,when dragging the bottom edge of window,the ball becomes an ellipsoid
After clicking the windows,it is restored as ball.
How to fix this issue?
It would help to know what you are doing by posting some sort of example. I am new to using libvtk, but would suggest you explicitly define your Camera etc. and dump the vtkCamera GetViewTransformMatrix() , vtkWindow->GetSize() and vtkWindow-> GetActualSize() (“returns the size without multiplying with the tile scale”).
If the defined transform and tile scale are the same before/after the window resizes, than it is likely a Qt or driver related issue (in other libs like wxWindows I’ve seen similar implicit “stretch to fit” issues with software/opengl drivers/debuggers that do not handle the each platform specific environment the same way.)
Tip: developers can’t read minds, 
and posting screen caps along with a minimal example can help form a basis for a bug report, unit test, or documented platform compatibility issue (helps to include information like the OS, video driver setup, and lib version).
Cheers, =)
J