In high voltage direct current（HVDC） and AC inter-connected electric networks， blocking of both poles of HVDC system has severe impact on stability of the overall system. When such block of both poles fault occurs in the Yin-Dong HVDC transmission system， stability problems such as destabilization of ultra high voltage AC transmission line and system low frequency may be triggered. To ensure stable operation of the system under such circumstances， quick and proper control strategy to shed certain load should be taken. Based on research on control strategy of±660 kV Yin-Dong HVDC security and stability control system of Shandong side， specifi cally to electric network with huge HVDC power injection， practical research methods are proposed. These include effective optimal load-shedding techniques and HVDC blocking criteria for security and stability control system. The methods are based on the hierarchical control mode of main station and sub-station. The load shedding method follows the rounds and layers. The validity of the strategy has been justified by the application results in Shandong power gird.