The system indexes of a space-based synthetic aperture LiDAR (SAL) system used for ground imaging are analyzed. A membrane-based diffractive optical system with a 10 m aperture is chosen to satisfy the requirement of power aperture product. Based on the synthetic aperture radar phased array antenna model, the device parameters and beam patterns in the diffractive optical system are analyzed. Aiming at the aperture transition problem existing in the diffractive optical system with a large aperture, a signal compensation and focusing method with a high range resolution is proposed based on digital signal processing. The research results show that based on this space-based SAL with a 10 m diffractive aperture, the image tracking with a high resolution and a high data rate to the specific long-distance targets is possible. Moreover, the realization of this technique can be feasible.