Magnitude simulator is the calibration and critical testing equipment on the ground for star sensor. A collimation optical system is designed to simulate the "infinity" starlight. In this system, four lens separation structure is used to meet the demand of weak light and improve the quality of image. The beam passes through the external exit pupil to the star sensor, and high-precision star image simulation is achieved. Based on the pulse width modulation (PWM) signal, a single-point controllable LED light is adjusted as the high-precision variable light source. With a contrast of 631∶1, the single-point control and joint control functions of the light source system are realized. The magnitude is tested in a darkroom. Results show that, under 10 μm star hole, the system can simulate 0-7 star magnitude. The magnitude of the tuning range is not less than half of a star, the simulation precision is less than 0.1 (the points of repeat tests fall within ± 0.4% of the required range), and the spectral range covers the entire visible spectrum.