A spatial optical detection system with a large field of view is designed. The parameters of this system are calculated by combining the spatial target characteristics and the performance indexes of the detector. Thus, the technical indexes of this system are determined, and the detection of the ninth-magnitude star target 255 km away in space is realized. This system adopts an off-axis three-mirror optical structure with a spectral range of 400-900 nm, a focal length of 64 mm, and a field of view of 30°×30°. The primary and tertiary mirrors in this system are designed based on the Zernike and XY polynomial freeform surfaces. The ray tracing is performed and the discrete point data are obtained. These data are fitted by MATLAB to obtain the profiles of the freeform surfaces of these mirrors. This system has a high energy concentration and a good imaging quality.