In order to analyze influence of the external factors to the tracking precision of space-ground optical communication,the platform vibration,atmospheric turbulence,and background light are studied,and can be attributed to the influence of tracking precision.Tracking simulation system is founded on the basis of measured platform vibration data.The compositive influence of various factors to tracking precision is analyzed under different conditions.Based on the assumptions,coarse tracking can well suppress the vibration of mobile platform on ground and ensure that laser beam can enter fine tracking field;the middle atmospheric turbulence increases the standard deviation of fine tracking error to about four times;if low altitude area is selected as examination spot,atmospheric turbulence will increase standard deviation of fine tracking error to about two times.