Stray light analysis results to a three-mirror off-axis space telescope was present in this article with the measurement of PST (Point Source Transmittance ). The entity model of the system was built to determine the first and second order scattering paths. The result was calculated by the improved Monte Carlo raytrace method between 20° off-axis angle. It was demonstrated that the stray lights mainly composed with the first order scattering
and was coincident closely with the structure of the optical system. The PST equaled to 3.56 and 4.02 at 0.1° off-axis angle
6.63×10-5 and 4.48×10-5 at 20° off-axis angle. Comparing with the two-mirror off-axis telescope
the stray light of three-mirror system was graveness at the big off-axis angle
and could be reduced by changing the mirror’s scatting rate. The result indicated that it could meet the requirement.