With the increasement of the harmonic quantity in power system ,single phase harmonic power flow calculation can't meet the needs. This paper discusses the components model in detail under harmonic condition in three phase distribution power system, include generator,transformer,line and load. Particularly, the transformer model considers the winding connection, the phase shifting among primary and secondary windings, and the off-normal tapping.The three-phase model of transformer, used in traditional harmonic power flow, may the preuent the model from converging, so the paper proposes a three phase back forward sweep harmonic load flow algorithm based on phase-components. At last, a test example was used to prove the algorithm. The results indicate that the proposed method is efficient.