Abstract: Overlap factor is a key factor that affects atmosphere detection of lidar system at near range. The accurate calculation of overlap factor is helpful to obtain more accurate detecting results. To this end, a simulation method was proposed based on ray-tracing technique to obtain the overlap factor of non-coaxial lidar system. Firstly, using a ZEMAX optical simulation software, the ray-tracing was carried out on the transmitter and telescope receiver loaded with mechanical structures to calculate the laser intensity distribution and the field of view (FOV) function of the telescope at a given distance. Then the ratio of the FOV-weighted laser intensity to the total intensity was calculated, obtaining the overlap value at the given distance. Finally, the full overlap profile was fitted through the overlap values at different distances. And this method was applied to simulate an overlap profile for a specific lidar system, and analyze the impact of axis displacement, optical-axis misalignment, laser divergence, the FOV of the telescope, and mechanical obstructions on the overlap profile of the system. The results show that when the deviation angle between the optical axes of the laser and the telescope is equal to half of the difference between the FOV of the telescope and the divergence angle of the laser, the overlap factor cannot reach 1 in the far-field, which puts forward higher requirements for the design and installation of the lidar system.