Abstract: The study of large power outage accidents at home and abroad shows that most of the disaster accidents in modern power grid are caused by cascading failures, which represent a chain of Cascading Trips of a series of components, and large blackouts are small probability and big impact events. In recent years, the DC transmission project has developed rapidly in our country. The multi infeed DC transmission system with the end to the same AC network has been formed in the power grid, and a number of super large scale multi infeed AC and DC hybrid 2 have been formed in our country. The fault of AC and DC system may lead to the failure of commutation at the same time, which has a serious effect on the stability of the system. At present, a number of DC simultaneous failure events have occurred due to the fault of the AC line. With the increasing of the size of the multi infeed DC, the decrease of the electrical distance between the DC drop points and the failure of the multiple DC commutation at the same time, there is a possibility of an avalanche cascading failure to prevent the occurrence of large blackouts. Tracking the operation mode of the power grid and searching for the AC and DC cascading failures that may cause multiple DC and phase commutation failures online are of great significance for guiding the operation of the power grid. The current research methods mainly use simulation methods and can not cover the whole communication system, and do not consider the mutual influence of multiple DC commutation failure. When the single DC commutation failure is failed, the active power and reactive power of the DC transmission power grid will be disturbed, and then other DC commutation will be triggered. Failure. For this reason, this paper proposes a fast search method for AC fault region, which takes into account the interaction of AC and DC interaction and multi DC interaction, and then evaluates the impact of single DC or multiple DC commutation failure and latching on the receiving end grid. Based on a fast search method of DC commutation failure caused by AC fault based on the multiple infeed DC short circuit ratio, the two port de Venan equivalent of the power grid is carried out from the fault bus node and the converter station node. The influence of the AC line fault on the DC converter station node voltage is calculated quickly, and the minimum turn off angle standard is derived. AC failure criterion that causes DC commutation failure. At the same time, the influence of DC commutation failure on other DC and the influence of AC fault on DC are also taken into account. From the fault bus node, the commutation failure DC converter station node and the adjacent converter node, the multi port de Venan equivalent of the power grid is carried out to calculate the reactive power impact of the DC commutation failure on other DC converter stations. The influence of bus voltage is derived, and a comprehensive criterion for simultaneous commutation failure caused by AC faults is derived. Then the impact of DC commutation failure or locking to the receiving end power grid is put forward, and the quantitative evaluation is made from three aspects of the active power impact, the reactive power impact and the transfer of the power flow. The simulation of the actual power grid shows that the identification method of the important lines is more comprehensive, and the construction method of the AC / DC multiple fault sets is practical.