Large-scale grid-connected power supply based on inverters brings severe challenges to the stability and security control of power system. Due to the simulation resources and calculation efficiency, it's necessary to equivalent the multi-inverter power supply in the new energy field when conducting the system stability and security simulation analysis. In this paper, for the scenario where multiple inverters of the same type are connected to weak grid, the open-loop transfer function and output impedance of the inverter grid-connected current are derived based on frequency analysis, and the stability judgment based on the impedance model in weak grid is obtained. Furthermore, the transfer functions of the inverter single equivalent and multiplication equivalent solutions are derived. The stability differences and key influencing factors are analyzed between the two equivalent solutions and the actual parallel multi-inverter system. Finally, the results of the theoretical analysis are verified by simulation based on an example. Through theoretical and simulation studies, the differences in the principles, effects of different equivalent schemes and the applicable conditions are clarified, and guidance is provided for reasonable selection of inverter equivalent solutions and simulation analysis.