Abstract: The wireless power transfer(WPT) system with relay coil has a longer transmission distance. And this technology has better practicability in coal mines. However, the special environment in coal mine has a high demand for the coil size. So the WPT system with asymmetric coil structure is suitable for the application in coil mine. Aiming at this problem, the WPT system with asymmetric three coils structure is proposed. And combined with the transmitting coil radius, the receiving coil radius and the transmission distance, the influence of relay coil position and radius on the transmission efficiency is analyzed. The influence law of relay coil radius on the transmission efficiency with different parameters is obtained. The results show that with the increase of the relay coil radius the transmission efficiency increases first and then decreases. And the law is independent of the transmission distance and the radius of the transmitting and receiving coil. However with different parameters the relay coil radius corresponding to the optimal transmission efficiency is different. Finally, an experimental platform of the WPT system with asymmetric three coils structure is designed. And the experimental results verify the correctness of the theoretical analysis.