The power rating for Electric Vehicles wireless charger keeps increasing as the technology develops. With limited space and volume, the maximum transferring power has reached a bottleneck even with an optimized coupling winding design. It is well known that third harmonic injection can increase the maximum power transfer for line-frequency inverter. However, for high frequency resonant converters, it is a big challenge to utilize third harmonic as the resonant network is a band-pass filter for fundamental frequency only. Also, the switching frequency is close to resonant frequency and the phase relationship between fundamental and third harmonic mostly relies on impedance match. This paper explores the feasibility of third harmonic injection in resonant converters, and theoretically proves the additional third harmonic can lead to a maximum 40% extra output power. A W-filter which is low impedance for both fundamental and third harmonic is proposed. To adjust the phase relationship, a dither feedback across the nonlinear output diode rectifier is proposed. A minimum system and a wireless power transfer (WPT) system have been constructed by W-filters in simulation. Experimental results based on a 20-W scale-down prototype are presented to illustrate the proposed system.