Based on the analysis of conventional two-coil resonant structure, a three-coil resonant structure is proposed to enhance the transmission efficiency for wireless power transfer system. The mathematical model of both two-coil resonant structure and three-coil resonant structure are deduced, and the transmission efficiency are compared and analyzed. It is concluded that the transmission efficiency of three-coil resonant structure is greater than that of two-coil resonant structure at a relatively long distance. Finally, an experimental platform, based on the theoretical requirements, is built to evaluate the system performances. The experimental results are provided to verify the correctness of the proposed method.