Spectral collinearity and limited spectral datasets are the problems influencing Chemical Oxygen Demand(COD) modeling. To address the first problem and obtain optimal modeling range, the spectra are preprocessed using six methods including Standard Normal Transform, Savitzky-Golay Smooth Filtering(SGSF) etc. Subsequently, the 190-350 nm spectral range is divided into 10 subintervals, and Interval Partial Least Squares(IPLS) is used to perform PLS modeling on each interval. The results indicate that it is best modeled in the 7th range (238~253 nm). The values of Mean Square Error(MSE), Mean Absolute Error(MAE) and R2score of the model without pretreatment are 1.6489, 1.0661, and 0.9942. After pretreatment, the SGSF is better than others, with MSE and MAE decreasing to 1.4727, 1.0318 and R2score improving to 0.9944. Using the optimal model, the predicted COD for three samples are 10.87 mg/L, 14.88 mg/L, and 19.29 mg/L. To address the problem of the small dataset, using Generate Adversarial Networks(GAN) for data augmentation, three datasets are obtained for Support Vector Machine(SVM) modeling. The results indicate that, compared to the original dataset, the SVM"s MSE and MAE have decreased, while its accuracy has improved by 2.88%, 11.53%, and 11.53%, and the R2score has improved by 18.07%, 17.40%, and 18.74%. The data enhancement method based on GAN network has important research significance and application value in the field of spectral analysis.