Acquisition, tracking, pointing (ATP) system is one of the key components for intersatellite laser communication. After acquiring, the accuracy of pointing and tracking is an important performance index of ATP system. Because many factors restrict, in fact, the ATP system can only attain limited precision. The optical signal beam divergence angle seriously affects the performance of intersatellite laser communication, so the choice of it for the entire system is very important. Three influencing factor of beam divergence angle are put forward and an optimum beam divergence angle based on the analysis of ATP accuracy is given out. The optimum divergence angle consumes the minimum transmitting power to satisfy indexes of communication system.