The propagating laser in free space was imaged by charge coupled device (CCD), so the incidence beam angles information can be obtained for the transmitting laser beam control. In satellite optical communications, improving the real-time measure precision for the angle errors of the incoming beam, higher tracking performance of a terminal can be reached and the laser link can be maintained steadily. The principium of the beam angle errors measurement was analyzed, and a model was founded for the angle errors measurement capability evaluation of the tracking assembly in satellite optical communications. The relationship of angle errors measurement precision with the size of dispersive spot imaging on CCD was analyzed using the model, and then some experiments were implemented to validate the results. It showed that considering the requirement for angle measurement precision and condition of the terminal power limitation, the relative size of the spot imaging on CCD should be between 2 and 3 in the design of a satellite optical communication system.