Coordinated control of reactive power needs to be considered when a DC power system provides emergency DC power support（EDCPS） to improve system voltage stability. The ratio between active power transmission and reactive power consumption of high voltage direct current（HVDC） system is analyzed and derived. Then transient voltage dip near converter station caused by EDCPS among multi-circuit HVDC systems is studied. In order to implement reasonable distribution of reactive power， some strategies such as: emergency inputting spare reactive power， fully using reactive power compensation ability and coordinated control for active power between AC and DC system， etc. are proposed. Simulation results based on practical AC and DC power systems verify the effectiveness of the control strategies proposed.