E-class power amplifier can be the ideal high-frequency power supply due to the advantages of simple topology, high working frequency, high efficiency and so on. This paper analyzes the transfer characteristics of four-coil WPT system in detail and proposes the optimal design method of the high transfer efficiency. Meanwhile, taking the working status of E-class amplifier into account, the perfect matching of input resistance is achieved by adjusting the distance between driving coil and transmitting coil. A 2.81MHz four-coil mid-range WPT system adopting E-class amplifier is setup. When the transfer distance is 3.6 times of the side length of the transfer coil, the overall transfer efficiency from source supply to the load can be 8.5%.