In this study, a nonlinear 3-branch equivalent circuit model is established that considers the instantaneous dynamic characteristics of supercapacitors. A dynamic capacitor Cf is introduced to represent the effect that the terminal voltage has on the capacitance for supercapacitors based on the traditional model, which can improve the precision of the description of a supercapacitor's dynamic characteristics. Then, the parameters of the supercapacitor model are identified using Levenberg-Marqurdt method, and a simulation analysis and experimental validation are performed. The simulation and experiment results show that the linear and nonlinear 3-branch equivalent model can quickly track the measured voltage value under the dynamic working conditions of Economic Commission for Europe and Urban Dynamometer Driving Schedule. However, the nonlinear 3-branch equivalent circuit model has a smaller voltage error, and the minimum error is 2.16%. In summary, the proposed nonlinear equivalent circuit model can more accurately reflect the actual dynamic characteristics of supercapacitors with a better engineering application value.