In view of problem that the stochastic flow produced by the distributed power accessing to distribution network and load change can cause the possible problem of overload, this paper proposes a bidirectional power flow analysis method based on sequence operations. Firstly, the serialization method is used to describe the random variation of distribution network system that contains the distributed power. Based on that, combined with the circuit state transition model, a comprehensive risk evaluation of remaining available margin probability distribution changes of the positive and negative of the branch due to uncertainty output is made. Example analysis of IEEE-34 node system proves that the method can identify the sensitive and dangerous lines in distribution network accurately, and provide optimized decisions for safe operation of distribution network containing distributed generation effectively.