A mathematical model for the output waveform of SPWM VSI (voltage source inverter) is created specific to the harmonic problem of SPWM voltage source inverter. Through theoretical analysis and calculation, we can get the distribution regularity of the output harmonic wave in the process of frequency conversion and voltage transformation. Utilizing the matlab to verify the simulation, we can get such results like "frequency conversion will lead to the shift of the harmonic wave's distribution center", "voltage transformation will cause the valuable change of the harmonic wave and the valuable c...