Abstract: Switching power amplifiers are widely used in civilian and military applications, such as electric vibration test stands, communication systems, and sonar detection. An optimal switching frequency control strategy based on particle swarm optimization (PSO) algorithm was proposed for switching power amplifiers used in extremely low frequency electromagnetic wave communication. While satisfying a certain output voltage distortion rate, it can reduce the thermal swing of power devices under extremely low frequency conditions and extend the life of the switching power amplifier. Firstly, the degree of freedom of thermal stress control of the switching power amplifier was analyzed. Then the important factors that affect the device lifetime——the thermal swing and the index that characterizes the output accuracy——total harmonic distortion (THD), the influencing factors of these two variables were studied. On this basis, the optimal switching frequency control strategy considering the thermal swing and THD were proposed, which is based on improved particle swarm optimization (PSO) control. Finally, the proposed control method was verified by simulation and experiment.