The mathematical model VSC-HVDC system in dq reference frame is derived.For the VSC-HVDC system of transmitting power to passive network ,a constant DC voltage is used in the rectifier-side;a constant AC voltage controller is used in the inverter-side.Digital simulation models are set up by MATLAB/Simulink. Simulation results verify that VSC-HVDC has a independent control of active and passive power,and demonstrates superiority of transmitting active and passive power simultaneously when connecting to passive network.