A method for detecting a parfocality of a zoom-stereo microscope includes: acquiring four highest definitions corresponding to a plurality of images with a cooperation of four definition judging functions, acquiring a relatively clearest position according to the four highest definitions, comparing a definition in the relatively clearest position with a definition in a parfocal position to judge whether the relatively clearest position is the parfocal position, then adjusting a magnification of the zoom-stereo microscope to acquire the parfocal positions at a finite number of the discrete magnifications, and finally fitting a parfocal curve at the continuous magnifications. The method according to the present invention implements a parfocality detection of the stereo microscope automatically and effectively and increases a productivity, and has a high detecting precision. In addition, the method according to the present invention has a good robustness, so that users needn't intervene and adjust frequently.