To reduce the residual attitude error of the drag-free satellite， a pointing stability control mechanism based on momentum exchange was designed. To realize nanoradian-level pointing stability accuracy for the satellite， a composite axis control method was implemented. This method work in conjunction with the drag-free attitude control loop， simultaneously reducing the consumption of working fluid in other actuators. This approach was designed to extend the satellite's operational lifespan. Firstly， the design incorporated a linear reciprocating mechanism driven by piezoelectric ceramics on the satellite. This setup allowed for the derivation and analysis of additional interference torque， which resulted from the movement of small masses. The analysis was grounded in the principles of satellite attitude dynamics. Secondly， the composite axis system's main and auxiliary axes were individually tuned， and the system's bandwidth was calculated to ensure it meets performance requirements. Following this， a feed-forward inverse model and adaptive PID control were employed. These techniques compensated for the nonlinearity of the piezoelectric ceramics and any disturbances affecting the satellites. Finally， the feasibility of the overall system was verified by an Adams and Simulink joint simulation. The simulation results show that the intervention of the mechanism can effectively reduce the bandwidth of the control system and realize high-precision pointing stability control of approximately 10 nrad·Hz^-1/2@1 mHz~1 Hz.