Presently in transformer model, it is considered mainly whether the equivalent impedance is assigned to the side of normal ratio, but seldom whether the equivalent impedance is on the side of on-load tap changer. The calculation results show that there is much difference between the model whose equivalent impedance being on the side of on-load tap changer and the one on the side of fixed tap, and this conclusion is also proved by theoretical analysis. Actually, both leakage reactance and resistance of a transformer winding varies with the tap position, so it is necessary to choose an appropriate transformer model which can reflect this relation. The analysis on equivalent impedance of the transformer model indicates that the transformer model with its equivalent impedance being on the side of fixed tap is rational one.