Abstract: In view of the problem that lots of surplus power surges in the sending Grid and the AC Grid splitting caused by the commutation failure or restart of HVDC system, influences between the DC faults caused by the sending Grid fault and artificially setting DC faults are compared, and it is found that safety and operability can both be gained by twice artificially setting DC and scanning AC fault at sending grid substituted by the commutation failure. The power impact of different power combination faults under total constant power on the AC section at sending grid is also compared, so the sum of DC power can be used as a grading standard. The impact of location and capacity of plants at sending grid on DC surge is analyzed, and grading control strategy of the capacity of plants at sending AC grid is put forward. In this paper, PSASP software is used to analyze the constraint of AC transmission section when the high voltage DC system operates with full power and the demand of the high voltage DC system to ensure the current transmission limit of AC section. Pre-control strategy is put forward for AC section to improve the coordinated AC/DC transmission capability based on the security of power grid.