For most uncooled infrared imager, there locates a baffle between the window of detector and the last lens of the optical system to block the internal stray radiation produced by lens cone and other structural parts. On the other hand, the baffle itself also brings another infrared radiation, and it has long been identified as a serious issue. Optimizing the surface shape of the baffle by modeling and calculating the distribution of stray energy on image plane is necessary to minimize the effects of the scattered radiation on the focal plane array (FPA). The Monte Carlo (MC) method has been verified to be an effective ray tracing technology in the computation of stray light, but for the baffle with complex heterotypic surface, the calculation by this way is very complicated and costs much time. Based on previous studies, this paper will present a MC method to trace the amount of rays radiated from the outer surface of detector, scattered by baffle and directly transmitted back to the FPA. Compared with the conventional MC rays tracing method, the way of spatial mesh discretization and gradual mesh reduction is proposed to replace the way of solving equations to search the intersection point between rays and complex surface on the baffle. As a result, it has higher computational efficiency and applicability to different shape of surfaces. Accordingly, it can be applied to the optimization calculation of baffle’s surface structure.