As a high-frequency inverter circuit, the class-E inverter has the advantages of simple topology and high inverter efficiency. However, because of its sensitivity to the load, it is easy for it to work in the hard switching state because of a load change, which causes system efficiency to be reduced. In view of this characteristic, an improved class-E inverter with a parallel oscillating branch is proposed. When the voltage across the switch tube is negative, the capacitor current is drained and oscillated through the shunt branch connected in parallel to the reverse conduction, thereby achieving zero-voltage switching and zero-derivative switching under non-optimal load conditions. Finally, based on the theory, an experimental simulation is carried out, and an experimental prototype with a switching frequency of 50 kHz is built. The efficiency of the inverter is 97.39% under non-optimal load conditions. The results verify the correctness of the improved class E inverter theory. This work is supported by National Natural Science Foundation of China (No. 51777120).