The invention relates to a PWM modulation method for improving harmonic characteristic of an output current of a three-phase photovoltaic inverter. According to the method, a sine SPWM modulation wave is injected with third harmonics with smooth waveform to obtain SVPWM modulation wave; the SVPWM modulation wave is adopted for PWM modulation; and the waveform peak value slope in the third harmonics is close to be zero. By adoption of the PWM modulation method, the problem caused by sudden change of the common-mode voltage slope can be solved, the harmonic characteristic of the output current can be improved, and a better modulation effect can be obtained.