The three-phase model of the transformer is especially important for power flow calculation in distribution system. In view of the problem that the traditional incidence matrix method is not suitable for the transformer with ungrounded wye connection, the three-phase transformer model with neutral parameters is proposed, which can be derived by incidence matrix method. This model can be applied in the power flow to calculate transformer neutral voltage directly. When the transformer model with ungrounded wye connection contains neutral parameters, the three-phase transformer model of all connection groups can be deduced by incidence matrix and the transformation matrix, which makes the derivation of the transformer model more concise. The results of the IEEE-4 node test system verify the correctness and effectiveness of the proposed transformer model.