The operating characteristics is determined by the controller parameters in the VSC-HVDC, in order to make full use of the fast and controllable characteristics in the VSC-HVD, this paper takes advantage of the improved PSO (particle swarm optimization) algorithm to optimize the outer ring PI parameters of the rectifier side controlled by constant active power. First, the small signal model of the rectifier is established. Then the equivalent transfer function of the inner loop is deduced. Besides, optimization are done to the outer loop PI parameters based on the deduced equivalent transfer function in the MATLAB. Finally, compared with the original parameters in the PSCAD/EMTDC, the simulation results show that the transient performance based on the proposed optimization method has been improved significantly, when the system is running at different operating points, the response characteristics are improved obviously; also, the system has strong robustness.