Abstract: When the wireless power transfer（WPT）system operates in an over-coupled state,the efficiency becomes low because frequency split phenomenon occurs.The expression for the relationship among the efficiency,the input impedance and the operating frequency is derived by the equivalent circuit theory（ECT）.Frequency splitting is analyzed and the effects of key parameters on frequency splitting and efficiency are obtained.Then a method of adjusting the parameters is proposed.The Ltype impedance matching network is used to adjust the equivalent load resistor.Frequency splitting phenomenon can be suppressed and the efficiency can be improved by the proposed method.Finally,a magnetic coupling resonant WPT system is designed and created.Simulation and experimental results have verified the validity of the theory.