The access of large-scale distributed generators will impact the reliability of the distribution networks,which might be out of range by the traditional fault diagnosis methods. In the distribution networks with inverter interface distributed generator (IIDG), the fault current will change significantly due to the fast response of controllers.Therefore, in order to improve the comprehensive reliability, this paper extends the traditional fault analysis method, so that the root mean square (RMS) of fault current from theIIDG and the grid detection current can be calculated accurately forfault analysis. Through the simulation inthe IEEE standard distribution networks, the accuracy of the proposed methodunder different fault conditions is proved. This method can be applied for improving the stability, the reliability of distribution network operations, and the automation level of distribution networks.