对有中间像的长焦距离轴三反系统的杂散光特性进行了模拟分析，并提出了相应的杂散光抑制方法。通过正追和倒追的分析方式，确定系统杂散光的主要来源，找到系统的重要面；根据分析结果，提出了杂散光抑制措施，用系统重要面的变化定性评价杂散光的抑制效果。分析结果表明，采用抑制措施后很好地抑制了系统的一次散射杂散光。用点源透过率(PST)定量评价光学系统的消杂散光能力，分别计算东西、南北方向离轴角为±40°的PST值，并结合光机系统的应用环境对系统的杂散光抑制能力进行了定量分析。

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.