The forced commutated voltage source converter (VSC) is becoming an interesting alternative for HVDC transmission, in this paper, the model of VSC based HVDC is established, and the model is applied to multi-machine power system including VSC based HVDC, a decoupled control strategy based on the established model is proposed, and the performance of HVDC in multi-machine power system is studied under three phase AC fault, validity of the proposed controller is verified by the simulation results.