To investigate the impact of ghost images on optical system performance, we develope a Modulation Transfer Function (MTF) calculation model under the influence of ghost images generated by secondary reflection. We first introduce the analyzing and describing method of ghost image using the paraxial approximation. Then, starting from the definition of the MTF, and considering the influence of ghost image irradiance on the modulation of the image plane, an MTF calculation model under the influence of ghost images is constructed. By conducting a case study calculation on a system and comparing it with the simulation results, it is found that the maximum mean square error is less than 0.049373, which verifies the accuracy of the model. Furthermore, a detailed analysis is conducted, examining cases that exhibited larger errors and clarifying the range in which this calculation method can be applied. The research results indicate that the paraxial approximation method is both accurate and reliable when calculating the MTF under the influence of ghost images is accurate, and is applicable in most cases. This study serves as a valuable exploration in the ghost image analysis of optical systems.