In this paper, in order to solve the problem that the existing star simulators cannot simulate the sky background change, a star simulator with variable background is proposed to test the performance of the star sensor. Off-axis reflection collimating optical system is designed, by introducing a semi-inverse semi-lens in the main optical path to produce two conjugate focal planes, the star-point reticule and the stray diaphragm are illuminated by two integrating spheres in the stellar source simulator and the background source simulator, respectively. Real-time monitoring of the irradiance at the outlet of the integrating sphere, and the output power of the light source can be controlled according to the needs of star and background brightness. After the two beams merge, they become parallel light through the collimating optical system and are finally received by the star sensor, thus realizing the simulation of stars with variable starry sky background and apparent magnitude. The test results show that the apparent magnitude of the system can be adjusted continuously from -2.00-6.00, the apparent magnitude simulation accuracy is 0.30, and the uniformity of the background light source is 97.3%. The system can well simulate the background changes of stars when the star sensor is running, and can effectively improve the performance test accuracy of the star sensor.