This paper proposes a design method of class-E inverter for the mega-hertz wireless power transfer (WPT) systems. It realizes zero voltage switching (ZVS) and reduces the peak switch voltage of inverter with load variation. Moreover, the design of four-coil WPT systems considering voltage limitation of compensation capacitors is also studied, and a prototype is built to verify the methods proposed. The peak voltage of switch decreases to about 3.4 times of the input DC voltage of class-E inverter, and varies less than 5% with load variation. With the coupling coefficient of 0.078 between transmitting and receiving coils, the system efficiency reaches 83.5% at the rated output power of 60 watts, and the inverter efficiency reaches 96.0%.