Abstract: Multi-level inverters are widely used in medium- and high-voltage high-power systems due to their low harmonic content, small electromagnetic interference, high voltage and large capacity, and good output waveform quality. Selective harmonic elimination pulse width modulation (SHEPWM) is a common switching strategy in multilevel inverters, which can eliminate a specific harmonic compared to high-frequency modulation, and has the advantages of lower switching frequency and switching losses. Aiming at the problem that total harmonic distortion (THD) cannot be optimized in traditional SHEPWM technology, we derived an analytical expression of the total harmonic content of the output voltage, used the multiple population genetic algorithm to solve the analytic expression, and obtained the minimum THD value under the full modulation index while obtaining the optimal switching angles. Taking the three-unit 7-level cascaded H-bridge as an example, we presented the analytical model of THD and its switching angle trajectory diagram, and compared with the traditional multi-band SHE. It is found that the total harmonic content THD is greatly reduced. At the same time, the problem of current backflow in traditional multi-band SHE can also be solved. Simulation analysis and experimental results verify the feasibility and advantages of the proposed method.