With the rapid development of new power systems, more stringent requirements for the stable operation and condition analysis of transformers arise in the wide frequency range. Therefore, for the accurate consideration of the transformer equivalent model, we need to consider both the saturation effect of the transformer in the low-frequency range and the frequency-variable effect in the high-frequency range. This paper firstly introduces the equivalent circuit model of transformer based on dual principle, and the usual dual model lacks the consideration of the frequency variation effect for the winding structure, therefore, based on the topology of transformer, the equivalent frequency variation circuit model is obtained by the dual principle, through the construction of the transformer’s core and winding using different frequency variation models based on the dual principle, and the impedance frequency variation obtained by the analytical solver for electromagnetic field is compared, and it is verified that in the critical frequency range of the transformer, the frequency variation effect is not considered in the equivalent circuit model. The accuracy of the transformer impedance within the critical frequency reference point is verified by comparing the transformer core and windings with different dyadic-based frequency-variable models. In addition, the full circuit model of the transformer is simulated to verify the accuracy of the equivalent model of the transformer.