We propose a dynamic beam waist adjustment scheme based on Marcum Q-function to solve the problem that the inter-satellite optical communication is deeply affected by pointing error. In the absence of any approximation, a closed-form expression for calculating the state of the instantaneous channel is given by Marcum Q-function. The optimization model is set up and a simple algebraic solution for the optimal dynamic beam waist radius is derived by the model, when the instantaneous direction error angle is known. A new dynamic waist adjustment scheme is given, which can effectively reduce the effect of the pointing error. Numerical results show that the system performance with dynamic beam waist is obviously better than that with fixed beam waist, and the proposed scheme here can improve the performance of bit error rate and outage probability to a certain extent.