Since the detecting area of any photodetector can't be zero，in reality the photodetection for an optical pattern is always an area-detection. If the intensity distribution to the left and the right of the characteristic point of the edge in the detected optical pattern is unsymmetrical then a mistake wil1 arise from this kind area-detection for the point position lead in detormining the position of the edge, and thus will produce the edge checkout error. But by means of the dynamic simulation for the photodetection of the optical pattern，the edge checkuout error may be previously calculated. The results of the simulation may be immediately used for the correction of the error. In this paper we take the photodetection of the Fraunhoffer diffraction pattern from a circular aperture for example，introduce the method of the dynamic simulation for the photodetection of the optical pattern.