Abstract: In order to analyze the transmission efficiency of wireless power transmission system via electromagnetic resonant coupling with relay structure， the relay system model was set up and analyzed with couple mode theory. In addition， the same power change rule compare with non-relay system was derived out. Take advantage of electromagnetic simulation software to make a system analyze with the magnetic field distribution of the power transmission system， and to analyze the circuit model of relay system to get the load power ratio， the motion as relay coefficient was proposed. In this way， the relationship between the load voltage ratio and relay coefficient was derived out. According to equivalent relation of load voltage， the model of relay coefficient was obtained， which could calculate reasonable relay coefficient. Finally， the accuracy of this model was verified by the experimental results.

Key words: relay structure, couple mode, energy exchange, load voltage ratio, relay coefficient