Abstract: With the advancement of energy transition, wide concern has been aroused for the influence of the declining inertia of the power system on the transient angle stability due to the large-scale wind power and photovoltaic power generation connected to the power grid. It does not conform to the nature of the problem when the qualitative influence of the generator inertia on the transient angle stability is analyzed from the perspective of the sending and receiving subsystems of the power grid. Instead, based on extended equal area criterion（EEAC）, the paper analyzes the influence of the change in the equivalent inertia of the complementary-cluster on the transient stability margin of the dominant image system. Due to the different impacts of the fault scenarios on the transient flow of the complementary-cluster image system and the interaction of the above two effects, the complexity of the influence of the generator inertia on the transient angle stability is caused. The law is discussed through the analytical analysis and numerical simulation based on the two-machine Hamiltonian system.