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.