In order to calculate the equivalent EM parameter of mixed medium, in this paper, we proposes a new method of impedance simulation based on an equivalent circuit. First, the relationship between electromagnetic parameters and capacitance, inductance of material is set up, and then the model that can describe real composite accurately is established. The equivalent EM parameter could be obtained by calculating the equivalent impedance of mixed medium. Comparing this method with the classical formulae and finite-difference numerical method, it is proved that it can be used to calculate the equivalent EM parameter of material. In addition, introducing thin films to simulate the factor such as surface effect, the result of calculation would match with experimental results perfectly. This method has more advantages than classical formulae and numerical method.