Abstract: The characteristic of "strong DC and weak AC" of the power gird is further presented as the HVDC projects are gradually putted into operation. The coupling and interaction between the sending-side and the receiving-side AC systems is becoming more and more significant, and thus the multiple parallel-operation HVDCs simultaneous commutation failure trigged by faults of the receiving side power grid is able to cause huge power impact to the sending and receiving side power system and bring a series of stability problems. If the grid stability margin is low, it can even cause the whole network to collapse. In order to expand the stable operating space of the sending-side grid, some security and stability control measures are likely to be adopted aiming to decrease the impact of the simultaneous commutation failure of multiple parallel-operation HVDCs. Based on the three-area AC/DC equivalent model, this paper firstly proposed several kinds of security and stability control measures aiming to improve the capability of the sending side system to maintain stable operation after multi-HVDC simultaneous commutation failure from the angle of mechanism analysis. And then adaptability of the control measures proposed was presented. Finally, the control measures are verified by the simulation results on the basis of the "North China-Central China-East China" power grid case system.