In order to realize the requirements of the sensitivity, detection timeliness, aperture and total length of an optical detection system in the space debris detection scenario, we design a space optical detection system with a large relative aperture and large field of view. Moreover, based on the detector index and target characteristics, we determine the design parameters and realize the detection of 12.5 magnitude stars. The system uses an asymmetrical double Gaussian lens group optical structure and has the working band of 450-850 nm, field of view of 20°, F number of 1.05, and entrance pupil diameter of 150 mm. We design the front surface of a lens in the system based on an XY polynomial freeform surface. The design and analysis results show that the diffuse spot of the system is within the 2×2 detection pixels, the surrounding full field-of-view energy ratio within the 2×2 detection pixels is larger than 86%, and the maximum distortion is less than 1.4% . The optical detection system possesses a compact structure, a reasonable aperture, good detection effect, high detection sensitivity, and strong timeliness. The performance of the adopted materials meets the conditions of space use and matches up with the optical processing capability. The designed detection system can be used for the accurate detection of space debris.