Abstract: In view of problems that transmission performance of three-coil wireless power transmission system would change with moving of relay coil, coupling mechanism of three-coil wireless power transmission system was researched. An equivalent circuit model of the wireless power transmission system was built which is based on three-coil structure, and basic characteristic formula of output power gain and transmission efficiency were deduced. Interaction mechanism of the output power gain, transmission efficiency and characteristic parameters were analyzed, and laws of the output power gain and transmission efficiency of the system were obtained. The wireless power transmission experiment system was developed which verifies correctness of the theoretical analysis.