Optical synthetic aperture (OSA) can greatly improve the spatial resolution of the optical system. However, due to its long manufacturing cycle, it is difficult and expensive to manufacture. In this paper, we propose a method for numerical simulation of OSA imaging system, which can simulate the image process of the system before the system is manufactured and thus can greatly reduce the manufacturing costs. Firstly, the relationship of energy on the pixel of image plane between OSA systems of different filling factor are analyzed. Then based on the characteristics of the OSA imaging system, imaging model of optical synthetic aperture is analyzed. Moreover, after those methods of simulating space variant image system and space invariant image system are given. At last, a method assessing of the quality of optical synthetic aperture system was given. Simulation results are presented to demonstrate the feasibility of the proposed technique, in terms of spatially variant and spatially invariant optical synthetic aperture system was achieved.