A set of beacon tracking servo system based on coarse-fine composite control was designed, which was aimed at demonstration test of space laser communication in atmospheric channel. Influence factors of tracking accuracy were analyzed and the impacts caused by atmospheric turbulence were researched in detail. A laboratory measurement system was constructed and the test about the effects of atmospheric turbulence on the optical spot tracking accuracy was completed. The results showed that the tracking accuracy was approximately linear with turbulent intensity increasing in weak-medium-strength atmospheric turbulence, which achieved spot tracking function well with a total system precision varied from 5 μrad to 15 μrad. An actual measurement for the tracking performance and accuracy of servo system was conducted in an air-air laser communication demonstration. The total tracking accuracy was no more than 15μrad and in accordance with the laboratory measurement system.