Abstract: As a large number of commercial inverters with unknown parameters are connected to the power grid, the problem of harmonic degradation in the wide frequency domain is serious, which affects the normal operation of inverters and other equipment.Based on online data and numerical analysis methods, the construction and solving of the matrix model of harmonic source and harmonic coupling admittance for the inverter are the key steps to solve this problem. This paper proposes a gray-box wide frequency domain matrix model and a solution method based on numerical regression fitting. Firstly, the specific form of the matrix model is derived by using the relationship between voltage and current at the port in the frequency domain. Secondly, the stepwise regression algorithm is extended to the complex domain to solve the matrix model parameters. Finally, the validity of the model and algorithm is verified by a grid-connected inverter simulation system in MATLAB/Simulink. The results show that the proposed gray-box wide frequency domain matrix model and its solution method can quantify the effect of the inverter as a harmonic source and the direct coupling and cross-coupling relationships between the background harmonic voltage and the grid-connected harmonic current, while provide a reference for analyzing the mechanism of harmonic degradation. The accuracy and solution time are better than other models and other multiple regression algorithms in the complex domain.