According to the feature of hybrid power supply with one phase, two phases and three phases in the medium and low voltage distribution system, this paper extends the traditional sequence components method and presents a multiphase power flow solution. In the approach, the transform relationship of sequence components among one-phase network, two-phase network and three-phase network is reformulated by using forward-backward sweeping algorithm (FBS). Furthermore, this paper uses complete sequence components model to solve out the singularity problem of admittance matrix appearing in particular transformer connections. IEEE 4~8500 nodes standard systems are tested. Simulation results verify that the proposed solution has good convergence characteristics, strong robustness and efficient calculation. It also shows the superior applicability in large-scale distribution system three-phase power flow.