Abstract: Stray light analysis is one of the key technologies for the optical system design. Based on the Monte-Carlo method, the analysis of stray light level for multitarget compounding hardware-in-loop (HWIL) system in software Tracepro was present. Simulation results showed that two critical parts of stray lights of HWIL system were derived: one part was the unused expanded beam, which hit on the boundary of main concave mirror; another part was self-thermal stray light, which was caused by self-thermal radiation of key elements when system was under room temperature. The dependence of stray light coefficient on the cooling temperature of key element was analyzed, and the stray light coefficient was less 2% for cooling temperature of 200K. The simulation results are beneficial to the design of multi-target compounding HWIL system for the missile.