Chemical oxygen demand (COD) is an important criterion for detecting the emission of pollutants and judging the quality of water. This paper improves the absorption spectrum compensation model for COD and turbidity mixed solution in the dual-wavelength spectral method based on the Lambert-Beer law additive principle. It compensates for the influence of turbidity on the absorption coefficient of the COD solution at 355 nm by the absorption spectrum coefficient of the mixed solution at 623 nm. This paper establishes a linear relationship model between the absorbance difference of the mixed solution at 355 nm and 623 nm and COD. The experimental determination coefficient R² of the model is 0.983 35, with a relative error of 3.5% and an average error of 0.7 mg/L. The design of the model is simple and easy to systematize, which is of strong significance for practical application.