An impedance matching optimization method based on BPF theory is proposed to solve the problem that the traditional magnetic coupled resonant wireless power transfer with complex loads is too complex to achieve the optimal impedance matching in practical application, which will affect the power transmission efficiency. In the first place, a model of magnetic coupled resonant wireless power transfer system which can be equivalent to a band-pass filter is established , the analysis process is simplified and the direct expression of impedance matching for wireless power transfer system is obtained; Next, based on the analysis of the effect of impedance matching on power transmission efficiency, the theoretical optimal impedance matching under complex loads is obtained, and the practical design constraints for optimizing power transmission efficiency in low quality factor coupled resonant system are revealed.Finally, simulation and experiment show that the optimal impedance matching design proposed in this paper can make the system more adaptive.