The identification of the key nodes of the power grid is one of the important contents of the safe and stable operation of the power system. At present, the analysis of the key nodes of the power grid is basically for a single node, without considering the joint influence of multiple nodes. In order to accurately identify the key groups of the multi-node, a method based on the combination of complex network theory and physical characteristics of the system is adopted, and the tunability and damage resistance of the power grid are considered for comprehensive evaluation. By constructing the network node efficiency matrix, based on the damage-adjusted steady state and making it meet the principle of minimum load shedding, the multi-node groups influence of different node combinations is simulated. The results show that the key nodes do not always exist in the key groups. The influence of the multi-node groups is not a simple superposition of the influence of a single node. The multi-node key groups account for a small proportion of the group. This method is of great significance to the identification, utilization and protection of multi-node key groups of the power grid, and can effectively guide pre-disaster protection and post-disaster recovery.