The VSC-HVDC has the characteristics of fast responding, active and reactive power decoupling and AC bus voltage stabilizing. Emergency control system with VSC-HVDC control can enhance the level of transient stability of power grid while reduce the conventional control costs of generator tripping and load shedding. The mechanism of improving transient stability by emergency HVDC power support based on Extended Equal Area Criterion (EEAC) is proposed. Based on the comparison of different kinds of HVDC power modulation, the difference control effect on power angle recovery between VSC-HVDC and LCC-HVDC is shown. The priority of different kinds of HVDC in the emergency control should be taken into consideration. A method to calculate the amount of HVDC power support and the emergency coordination control strategy considering both the priority and the amount of HVDC power support in different fault conditions are proposed.