The optical system with the fixed relative aperture can only clearly image objects within a limited depth of field. Once reflected twice by two optical elements, the object beyond the depth of field is focused on the detector target surface, and a exceed depth of field ghost spot forms, which greatly reduces the contrast. In this paper, Code V and LightTools are used to simulate and analyze the optical system, and the surfaces of the two optical elements are located which generate the ghost image of depth of field. The simulation results correspond to the generated phenomena. It proves that the simulation analysis method is correct and feasible,and can be used in other optical systems.