In order to make full use of distributed generators and reduce power loss under the condition of large scale power loss in distribution network

on the basis of idea of on-line islanding algorithm

we proposed the principles of islanding algorithm and introduced the mathematic model. We proposed a weight value setting method for the islanding algorithm. An optimal radial distribution network generation method based on Kruskal algorithm produces the optimal radial distribution network

by which the best islanding scheme is proposed by improved genetic algorithm. PG&E 69 nodes system is used to analyze the influence of load weight setting and compare the existing methods. The result shows that optimal radial distribution network can reduce the solution space

which is the basis of optimal islanding algorithm. Compared with heuristic

branch and bound and dynamic programming method

the proposed two-step method is more effective in quantity and value of the loads restored

which can be used in distribution network operation control.