以点源透过率(PST)为评价标准，对一个离轴三反空间望远系统的杂散光进行了分析，并给出了分析结果。通过建立系统的实体模型，确定了一次、二次散射路径，采用改进的蒙特卡洛法，对20°之间各离轴角分别进行光线追迹。对模型的分析结果表明，系统产生杂散光的主要原因为一次散射，与光学系统结构密切相关。离轴角为±0.1°时，PST分别等于3.56和4.02，离轴角为±20°时，PST分别为6.63×10-5和4.58×10－5。实验表明，通过加入遮光罩及减小镜面散射率可减少杂散光。与离轴两反望远系统相比，三反系统的杂散光水平在大离轴角时偏大1到2个数量级，但可满足使用要求。

Stray light analysis on a three-mirror off-axis space telescope was presented in this article based on the measurement of Point Source Transmittance(PST). The entity model of the system was established to determine the first and second order scattering paths,and then different off-axis angles in ranges of 20° were calculated by the improved Monte Carlo ray trace method.Results indicate that the stray light mainly is composed of the first order scattering, which is related closely to the structure of the optical system. Furthermore, the PSTs are 3.56 and 4.02 at ±0.1° off-axis angles, 6.63×10-5 and 4.58×10－5 at ±20° off-axis angles. Experiments also show that the stray light can be reduced by modifying the baffles and changing the mirror’s scattering rate. Compared with that of the two-mirror off-axis telescope, the stray light of three-mirror system is graveness at the big off-axis angle, but it can meet the design requirements.