In MADDE, I used the following commands:

cd qthello
mad qmake
mad make
mad dpkg-buildpackage

all these commands are here:
http://wiki.maemo.org/MADDE/Qt_Example

And I got a .deb file after these commands.

I just want to know if I can use the .deb file to install my app developed with qt.

thx.

Yep, you can use that as a basis for your further development. For further reading on qt-packaging you can as well check this wiki page or this blog-post about the packaging of a homescreen widget.

Daniel