In order to develop the theories and methods for load flow calculation of a power system with the Extended Sen Transformer (EST), an EST model suitable for a unified iterative power flow solution is proposed from the perspective of simplifying the power flow model of the EST. First, according to the working principle of the EST, the series and shunt windings of the EST are both equivalent to a series-connected branch composed of a voltage source and an impedance. As a result the EST becomes an equivalent circuit of a dual voltage source. Then, based on the solution idea of the unified iterative method, a steady-state power flow model of the EST is established considering the operation modes of the EST such as the voltage regulation, phase angle regulation and independent power flow regulation. Finally, MATLAB software is used to carry out a simulation for the modified IEEE 6-machine 30-bus system. The magnitude and phase angle of the node voltages obtained through the Matlab simulation and the proposed model can reach acceptable agreement, confirming the effectiveness of the proposed model. This work is supported by National Natural Science Foundation of China (No. 51567006), Program for Top Science and Technology Talents in Universities of Guizhou Province (No. 2018036) and Science and Technology Fund of Guizhou Province (No. [2019]1100).