In this paper, a lumped model of the common mode choke for common mode consists of resistance, inductance and capacitance is adopted to reflect its impedance character and the complex permeability of the core under different frequencies is extracted. The values of the components in the model are obtained by finite element software instead of calculating the values at every frequency. The impedance obtained from the software is similar to the result obtained by the impedance analyzer. With the method, the impedance of the common mode choke can be known in advance and can help to design the choke.