Abstract: Dynamic reactive power demand of HVDC system and its power recovery characteristics are studied under different operation modes and failure cases of AC power grid in this paper. Taking CIGRE test system and a practical HVDC system as study case, the support capacity change of AC power grid and power varying rules of converter at inverter side of DC system under different failure severity degree are studied, and the characteristics of simple multi-infeed system are analyzed. The characteristic points and their occurring time of the change law of inverter power are summarized, and a time-domain series model of HVDC power recovery at inverter side is built. In the end, the time-domain series model of DC power recovery, which represents detail HVDC model, is applied to electromechanical transient simulation. By comparing the results of electromechanical transient simulation and electromagnetic transient simulation, the feasibility of proposed model is verified.