Synthetic aperture ladar can achieve high-resolution imaging with a small aperture, and had important application value in space-based space target observation. But, there were some problems such as narrow beam width, limited coverage, and difficult target search and acquisition. In order to solve these problems, a space-based synthetic aperture ladar space target imaging system combining active laser and passive infrared was adopted. The system index was analyzed, and the imaging simulation results were given. The results show that, the system is lightweight through laser/infrared common aperture. The problem of depolarization and target/platform vibration is solved by using circularly polarized laser emission signal and orthogonal linear polarization dual channel receiving signal, together with along orbital interference processing. Target search and tracking were realized through infrared camera. Target ranging and velocity measurement can be realized by using 10 ns pulse width narrow pulse laser signal. Pulse width and bandwidth of the optimized binary phase shift keying coded laser signal were 10 ns and 4 GHz. 0.10 m real-time low resolution imaging and 0.05 m high resolution imaging can be achieved. The system design scheme has reference value for the development of space-borne synthetic aperture ladar.