As a dynamic power electronic device, 500kV unified power flow controller (UPFC) is applied to control the power flow of transmission lines and optimize the distribution of feeding power into receiving-end grid. Meantime, UPFC is able to provide system with instant reactive power support. In this paper, effect of 500kV UPFC on HVDC feeding power grid is conducted through electromagnetic transient simulation. AC/DC system, UPFC device and control system including reactive power response strategy are built detailed in PSCAD, which is according to the dynamic characteristic of UPFC shunt side. Results show that UPFC can improve the voltage stability of HVDC feeding system and relieve nearby DC commutation failure.