The problems of spot detection and tracking in fine tracking of space laser communication systems are researched. According to the characteristics of four-quadrant detector,the spot detection and tracking system is designed by using two band amplification,digital signal processing (DSP) system,fast-steening mirror etc. The principles of composition and design are studied in detail. A testing system is set up,which can prove by experiment that when the incident power is 1.3 μW,the position resolution of 1.6 μm,angle resolution of 0.8 μrad,and subdivision of 80 in photosensitive surface can be achieved. And the tracking precision is up to 2.1 μrad in one time variance at the conditions of high signal-noise ratio. With digital signal processing used as the core in data processing and control system,this system has many advantages such as small size,convenience to modify parameters,and engineering etc.