Abstract: In order to study how to obtain the beam that meets the need of laser applications including such as beam pattern,spot size and intensity distribution,the formula for the diffraction transform of Gaussian beam throughout a micro-circular aperture was deduced based on Kirchhoff diffraction formula.The beam pattern change and its influencing factors of Fresnel diffraction transformation of Gaussian beam throughout the micro-circular aperture were discussed.The simulations carried out with the MATLAB software show that this kind of method is reliable and feasible.This is conductive to the application of laser in the laser engineering,micro-optics and micro-opto-electro-mechaning system.