In this paper, based on the practical application requirements of simulating infinitely distant target stars in high-precision star sensor ground testing tasks, and in view of the practical problems of large volume, difficult installation and calibration of conventional star simulators, an opto-mechanical structure of off-axis reflector star simulator with large aperture, long focal length and self-collimation function is designed to simulate the parallel light emitted from stars at infinitely distant distances. The design adopts off-axis reflective optical design to avoid center blocking. By adopting the focal plane splitting method of the optical tube, the transmission of the optical path through the reflection and transmission function of the semi-transparent semi-trans spectroscope is realized, and the length of the mirror tube is compressed. Collimation performance of the optical system can be directly distinguished by the detector receiving the light image transmitted by the beam splitter, which is easy to install and convenient for later use and calibration. And the use of the jackscrew to adjust the posture reduces the difficulty of mounting and adjusting. The actual test results show that the optical machine system has an inlet pupil aperture of ϕ210 mm and outlet pupil distance of 2100 mm, whose wave aberration (rms) of the whole optical machine system is 0.030λ, can accurately simulate 0~7 magnitude stars, and the working band is 500~800 nm. The design meets the requirement of calibration technical index for high-precision star sensor, and provides a design method for the efficient and rapid development of star simulator.