Single-phase photovoltaic inverters are widely used in distribution systems, and studying their harmonic models is of great significance for harmonic power flow analysis, harmonic interaction effects, and power system stability. Firstly, by analyzing the interaction mechanism between the grid and the inverter, as well as the harmonic conduction on the AC- and DC-side of the inverter, the impedance formulas of the single-phase photovoltaic inverter under different harmonics are derived. Secondly, the frequency coupling mechanism of harmonic impedance is analyzed in conjunction with the control loop, and the harmonic coupling impedance matrix model and equivalent Thévenin circuit of the single-phase inverter under different harmonics are established. Furthermore, based on the proposed model, a harmonic analysis of the system is conducted, revealing that the model significantly simplifies the harmonic power flow analysis of the single-phase inverter, improves the calculation accuracy of the system's harmonic power flow, and can effectively assess the impact of connecting the single-phase inverter on power quality. Finally, time-domain simulations are conducted to verify the correctness and effectiveness of the established model.