HVDC auxiliary emergency power control can improve the transient stability of AC/DC power system. Compared with other control methods, HVDC auxiliary emergency power control has the advantages of quick response, large adjustment capacity and less investment, so it is the primary choice to improve the transient stability. Taking multi-infeed DC (MIDC) transmission system as the research object, based on the definition of the traditional power support factor, considering the influence of the electrical connection of the sending-end power grid on the power support, a new power support factor (NPSF) is defined. Based on the NPSF, multiple DC coordination sharing strategies are implemented when one of the DC faults occurs. Finally, MIDC is built on the PSCAD. Single DC auxiliary power control and multiple DC auxiliary power coordinated control were simulated, respectively. Rationality of the definition of NPSF is verified, and it is concluded that multiple DC joint power control is superior to single DC.