@aria_aa said in how too add an exciting ".ui" file to a "cpp widget application project"?:

What should I do?

clean up the build directory run qmake rebuild project

Should work, code looks good.

And another question:
what is #include "ui_window_no_000.h" and #include "ui_window_no_001.h" in cpp files?
there is no file such as ui_window_no_000.h or ui_window_no_001.h in project.

These files are automatically generated from your .ui files by uic. It's part of the build process in Qt, same as with it running moc to generate meta-object functionality.