Bymeans of Collins formula, based on the method that a hard-edge aperture function can be expanded into a finite sum of complex Gaussian functions , the app roximate analytical exp ressions are derived for the free propagation characters of Hermite-Guassian beams passing through hard edge aperture . Finally, numerical examples are illustra-ted withMatlab software. It is demonstrated that the free propagation characters are closely related to the beam param-eters, optical system parameters, the size of apertures and the distance of apertures. The results can be directly used in other apertures, and be applied to control beams and op tical system designing.