In the light of the asymmetric channel of intersatellite optical communication subject to pointing errors, an optimal signal input probability distribution can be found to maximize the mutual information of the link. Considering the aperture size of receiving antenna, beam width and the parameters of pointing errors distribution, the channel matrix of intersatellite optical communication is given, and the optimization model of mutual information is established, when the pointing errors obey Beckmann distribution. The Lagrange multiplier method is used to solve the model, and the closed expression of the signal input probability distribution is obtained when the mutual information is maximized. Through numerical simulation, the optimal signal input probability distribution, channel capacity and the difference of mutual information vary with the normalized decision threshold, beam width and the parameters of pointing errors distribution are analyzed numerically. The simulation results show that the mutual information between input signal and output signal under the optimal signal input probability distribution is obviously higher than that under the equal probability distribution. Using the results of numerical simulation, the optimal source input can be selected on the basis of known distribution of pointing errors, so that the intersatellite optical communication channel can achieve the maximum information transmission capacity.