Multi-parallel grid-connected inverter system is increasingly applied in distributed power generation systems. Due to the existence of grid impedance, the output current of the grid-connected inverter cannot be fed to the grid totally, and a part of it will circulate to other inverters connected in parallel under weak grid. And the stability of circulating current also affects the safety and reliability of the system. Also, the large circulating current will bring more power loss to system. This paper uses the average model of the grid-connected inverter to establish the Norton equivalent circuit of the multi-parallel grid-connected inverter system. On this basis, the generation mechanism of the circulating current is explained by the current separateness, and the relationship between the stability of the circulating current and the injected grid current is indicated. Finally, the accuracy of the proposed theory is verified by simulation.