In order to maximumly eliminate the main harmonic of the inverter output voltage, a method of determining modulation ratio is proposed. Firstly, the output voltage of the cascaded multilevel inverter under unipolar frequency multiplication is analyzed by using the double Fourier series theory, and the harmonic distribution of output voltage is deduced. Secondly, a method of selecting the SPWM modulation ratio is proposed to reduce the primary harmonic of the inverter output voltage under the condition that the amplitude of the fundamental wave is constant. Finally, the correctness and effectiveness of the proposed method are validated through software simulation and hardware experiments under different modulation ratios in the cascaded five-level inverter. This work is supported by Key Science and Technology Program of Higher Education of Henan Education Department (No. 17A470001).