qt - view geomview (.off ) file I want to open a .off file and view the content 3d model using libQGLViewer, qt-creator.

I don't know how to display the 3d model after opening its .off file.

QString fileName = QFileDialog::getOpenFileName(this,tr("Open .off model"), "/home", tr("*.off"));
From a quick gander through the examples it looks like QGLViewer is a thin wrapper around a opengl window using the legacy api. 

So what you really need is a way to load a .off file to render in legacy opengl. it looks like a very simple format, even simpler than .obj. So simple that IMO it is worth it for you to figure out on your own.