Based on the equivalent circuit model of a supercapacitor and the complex impedance synthesis，a impedance function for the supercapacitors was established through fitness optimization of experimental data. The supercapacitors characteristics，such as voltage，capacity，current and state-of-charge，were simulated using the impedance function. The result is effective for supercapacitors to be used on the simulation and optimization of energy management system in pure electric vehicles or hybrid electric vehicles.