While electric vehicles (EV) can improve driving range through wireless power transfer (WPT), changes in vehicle axle load or suspension vibration caused by road conditions can change the coupling coefficient between transmitter coil and receiver coil, leading to WPT system being far away from the maximum efficiency transmission point. To address the problem that WPT system efficiency deviates from the extreme point along with the fluctuation of coupling coefficient, a realtime estimation method of coupling coefficient for seriesseries (SS) topology is proposed, on this basis, a novel maximum efficiency tracking control scheme is proposed, which is regulating the DC link current of secondary side by PI feedback controller to maintain the highest efficiency of WPT system when coupling coefficient varies. Simulation and experimental results indicate the effectiveness of the proposed method.