The study on the shape effect of particles for measuring result is more important in the measurement of particles' size distribution, therefore, appearance of particles' shape distribution must be considered in the advanced measuring method. In the traditional measuring method, the shape varying of particles is not considered. In this paper, it is necessary. The microscope is not suited to review the micrometer particles in a great number, a diffraction image processing method is put forward, that is, a sample board receives the moving particles and made as diffraction sample, which is irradiation by Laser, the diffraction pattern produced by it is processing based on the data computing by PC, some models are used within it. At last, the shape distribution function can be made at one time for greater number particles.