The precise autotransformer model is crucial to analyze the characteristics of the transformer in the magnetic saturation region. In this paper, starting from the electromagnetic relationship of the single-phase three-winding autotransformer, the equivalent circuit of three-winding autotransformer converted to the high voltage side is derived. Then, the calculation method of linear winding leakage impedance parameter and nonlinear excitation impedance parameter is given. And the method of obtaining the nonlinear inductance of magnetizing reactance by using the magnetic equivalent circuit is introduced in detail. Then the method of using JA hysteresis model to realize the nonlinear inductance is given. Finally, a three-winding autotransformer model is built in PSCAD by FORTRAN language. The simulation results verify that the proposed model can accurately reflect the nonlinear characteristics of the autotransformer. This work is supported by National Natural Science Foundation of China (No. 51677068).