| In the traditional system of ICPT, to calculate the optimal matching parameters between the transmitting coils(Tx) and the receiving coils(Rx) of loosely coupled transformer coils, usually by system modeling to project the mutual inductance value between the Tx and the Rx and supplemented by a large number of experiments. It not only lack theoretical basis but also waste manpower and material resources in this way. Pointing to this problem, this paper proposes a design method, using by simulating and observing the change rules of current density, to obtian the optimal matching parameters between the Tx and the Rx. Firstly, the system decoupling equivalent circuit model, that primary and secondary coils are in parallel compensation(PP) structure of the single switch inverter ICPT system, is used to derive the relationship among the current density, mutual inductance (M), system transmission power and efficiency. According to the analyses above, based on the finite element simulation software, the Rx coil current density is simulated with the variation of the Tx coil inside and external radius. The Rx coil current density and the mutual inductance of the magnetic-coupled-coils are simulated with the variation of the Tx coil inside and external radius. Besides, the corresponding regularity between Rx current density and mutual inductance change is simulated. The simulation results show that the ratio of Rx to Tx in the optimal size of loosely coupled coils is consistent with the empirical values summed up by the enterprise experiments.