Island is a new operating mode when the distribution network is incorporated with distributed generations. Based on the importance of the load an optimization model of distribution network islanding scheme is established， in which the maximum equivalent load is taken as objective function. A solution based on depth-first search and breadth-first search technology is given. By considering the islanding operating mode， the paper improves the minimal path method of the traditional reliability calculation and makes it suit to calculate the distribution network with distributed generation. The simulation of IEEE-RBTS Bus 6 system demonstrates the efficiency and practicability of the mode. The calculating results indicate that the distributed generation connected to distribution network can improve the reliability of the distributed system.