Stray light characteristics of an off-axis three-mirror optical system with a middle image and long-focus was simulated and analyzed, and corresponding suppression method for the stray light was proposed.however, little is researched on the stray light analysis of this structure. In order to satisfy the stray light suppression requirements of modern optical instruments, a simulation analysis is executed for the designed optical system. By a forward and reverse ray tracing method, the main source of stray light was identified and an important surface of the system was found out. According to the analysis results, some corresponding suppression measures were proposed to reduce the stray light of the optical system. The effect of stray light reduction were evaluated by comparing the number of important surfaces before and after the measures. The analysis results show that the first-order scattering stray light has been well suppressed. Furthermore, the Point Source Transmission (PST) was used to evaluate the stray light suppression capability, and its quantitative analysis was impletmented by calculating the PST of ±40° off-axis angles in EW and NS,respectively.