Mac: other application can appear between a modal QDialog and the QMainWindow

In my project, a modal dialog will popup after a button clicked. When i clicked other application like a  Qt Creator, the dialog and mainwindow will be inactive and show behind Qt Creator. There is no problem since then, but when i clicked the dialog or the mainwindow, let the my qt application be active, a problem appears. the Qt creator will appears between the modal dialog and mainwindow, the Qt Creator don't show behind my mainwindow.