The correct analysis of system state and the working mode of distributed generation are very important to the microgrid in isolated island operation， so a fast power flow calculation with high efficiency is very significant. Based on Newton-Raphson method， a three-phase power flow algorithm was proposed for isolated island microgrid with distributed generation operating in droop control mode， whose power flow equation was simplified and computational efficiency was improved. On this basis， the limit problem of reactive power of distributed generation was also considered. Finally， the correctness of proposed method was verified in a mircogrid test system with 25-bus three-phase unbalance， whose results were compared with Newton trust region method. The results indicate that the proposed power flow algorithm has high accuracy and efficiency， which can be used as the basis for further research.