The fast steering mirror (FSM) is one of the core parts in the composite axis precision tracking system. In order to research the beam deflection caused by decoupling FSM rotation, the FSM rotation is decomposed into longitudinal and transverse movement, and the space rectangular coordinate system is established. Rotating the FSM longitudinally and transversely, the cardinal relationship of the angular variation of the reflection rays in the FSM coordinate system is established theoretically. Based on the typical models, the simplified model in the azimuth and pitch direction is constructed. In this way, the engineering calculation could be simplified, and the results are verified through simulation experiments.