The frequency characteristics of the inductor-capacitor-inductor(LCL) type Inductive Power Transfer(IPT) system are analyzed by establishing the system mutual inductance coupling model. As a result of the analysis, the soft switching frequency is varied with the load. Therefore, a closed loop frequency tracking method based on FPGA is designed. It is verified by experiments that this method can make the operation frequency of the primary converter track the soft frequency and ensures the switches work at Zero Current Switch(ZCS) mode.