Abstract: Integrating many inverter-interfaced distributed generators (IIDG) will impact the power system's calculation of short-circuit current and grid equivalence. This paper proposes a multi-port Thevenin equivalent method for an external grid with a distributed generator, and the influencing factors of equivalent parameters are analyzed. Firstly, the Thevenin equivalent circuit model and parameter expression are derived based on the theory of multi-port electrical networks. The influence of distributed generators on equivalent circuits is mainly reflected in the open-circuit equivalent voltage. Secondly, according to the formula of open circuit voltage calculation, distributed generators' influence factors and rules on open circuit equivalent voltage are analyzed. The distributed generator can be equivalent to the voltage-controlled current source, and its output current is affected by the independent voltage source, the equivalent port voltage, and the low pass control strategy of the distributed generator. Then, the effect of equivalent port voltage, the capacity of the distributed generator, and the strength of independent AC power supply on equivalent open circuit voltage is analyzed, and the variation curve of equivalent open circuit voltage with port voltage, the capacity of the distributed generator, and internal impedance of external power supply is obtained. The simulation results of the IEEE33-node power grid and 220kV high-voltage power grid verify the correctness of the equivalence method and the analysis of influencing factors.