Abstract: In view of problem of low frequency harmonic in step voltage waveform of output of cascaded H-bridge multilevel inverter, a combination method of selective harmonic control was proposed. On the basis of trapezium waveform modulation, the method uses combination method of equal area principle and harmonic injection, switching conduction angle for selective harmonic elimination can be easily obtained by simple calculation of trigonometry formula deduction and optimized, so as to realize harmonic suppression. Taking three-phase nine-level cascaded H-bridge inverter as an example, simulation and experiment were conducted, and the result shows feasibility and validity of the method.