In the interaction with power grid，the randomness and volatility of photovoltaic（PV）system will introduce certain harmonics and inter-harmonics to the power grid，causing such problems as sub-synchronous oscillation and power quality as well as，in case of serious，collapse of the grid. Aiming at the harmonic problem of PV grid-connected system，the influence of harmonics is analyzed from such three aspects as PWM modulation of inverter body，interaction between output side and the grid，and fluctuation of PV power output on input side in order to provide harmonic analysis basis for harmonic control and safe operation of power grid. First，the high-order harmonics caused by PWM modulation of grid-connected inverter is analyzed and the low-order harmonics introduced when the dead-time effect is considered. Then，the interaction effect of non-ideal grid on the harmonics of grid-connected system is further investigated based on the formation mechanism of DC ripple；At the same time，in case of three-phase unbalanced operation of the grid，the output current of the inverter will have a third harmonic component. After that，the effect of PV power fluctuation on system harmonics is studied from such two aspects as MPPT algorithm and environmental factors and the daily variation curve of the illumination intensity is used for simulation. The result shows that. the total harmonic distortion rate of grid-connected current decreases with the increase of light intensity，which provides basis for harmonic detection and control.