Abstract: For the all-movable rudder system as a component to control the flight attitude and adjust the flight direction, its dynamic characteristics play an important role for the normal operation of a spacecraft. In order to analyze the dynamic characteristics of the all-movable rudder with motor servo system and rudder shaft gap, a method for modeling all-movable rudder system based on flexible multi-body dynamics was proposed. The rigid-elastic coupling reduced-order model for typical rudder surface was established via Craig-Bampton method. The connection mechanism between the motor and the all-movable rudder surface was created by multi-body dynamics method. The modal-based generalized aerodynamic model was produced using doublet lattice method. The simulation results show that the predicted flutter speed by the proposed model is 1270 m/s, with a deviation of less than 2% compared with a commercial software, verifying the correctness and feasibility of the modeling method. The research shows that the presence of servo system would cause a large jump in the response of the typical rudder surface, while the presence of the rudder shaft gap would greatly reduce the critical speed of the rudder surface to generate limit cycle oscillations.