This work analyzed the influence of the position of relay coil on the output power and transmission efficiency in the three-coil wireless power transmission system. A transmission strategy using a DC-DC conversion circuit was designed to maintain the transmission efficiency at a high level by adjusting equivalent impedance of load without changing the placement of the relay coil. The experiments show that the proposed transmission strategy is not only suitable for the situation where the relay coil is fixed and load resistance is changed，but also can maintain high transmission efficiency when the load remains the same but the position of the relay coil changes.