In recent years, wireless charging has become a trend because it is convenient and safe. However, its efficiency can deteriorate when the distance between coils or the load varies. In this paper, an automatic $\pi$ - impedance matching method is proposed to obtain higher efficiency under dynamic-coupling or varied-load condition. The simulation results show that the efficiency can remain higher than 85% in varying transfer distances when coupling coefficient is over 0.3. For varied-load, the efficiency is higher than 80% when the load is over 20 ohm and reach around 88% as the load increases at the 6.78-MHz operating frequency.