According to the advantages of flexible dc system with abundant control methods and flexible and reliable regulation means, an adrc based on ac system decoupling control based on ac-dc power decoupling control is proposed to provide damping support for ac system. Firstly, the mechanism of damping torque provided by additional damping controller is analyzed. Then design the excellent robustness since immunity damping controller for the system to provide the damping support and stabilize voltage, a typical 4 machine 2 area system by PSCAD/EMTDC platform building, and has carried on the simulation results under different controller scheme, the results show that the additional damping controller based on the immunity control better suppress the system oscillation and improve the system stability.