The fine tracking is the core technology of APT (acquisition,pointing and tracking) system in the satellite-to-ground optical communication.The construction and principle of fine tracking system based on FPGA are introduced and analyzed.A novel adaptive fuzzy PID algorithm is introduced,in which the weights of PID parameters can be adjusted in real time.In a terrestrial free-space 16 km optical data link,the experiments of fine tracking are demonstrated successfully.The resuts show that the efficiency of fi...