Exploring the optimal axial position of the relay coil of the three-coil magnetic coupling resonant wireless power transmission system is very important to optimize the transmission performance of the system. To this end, the transmission characteristics of the three-coil MCR-WPT system are deeply studied, the maximum power and maximum efficiency transmission conditions of the system are obtained by applying the circuit mutual inductance coupling theory combined with the coaxial coil mutual inductance calculation method. Based on the above optimal transmission conditions, the electromagnetic simulation of the influence of the axial offset of the relay coil on the transmission characteristics of the system in a wide load range is carried out. The simulation results show that the optimal axial position of the relay coil transmission power and transmission efficiency is related to the load, and both shift to the receiving coil side with the increase of the load. When the internal resistance of the power supply is equal to the load, the system can obtain the maximum transmission power when the relay coil is located in the middle of the coupling mechanism, and the system can obtain the maximum transmission efficiency when it is close to one side of the transmitting coil. Finally, a three-coil MCR-WPT system experimental platform is built, and the experiment verifies the correctness of the theory and simulation.