Differential Optical Absorption Spectroscopy (DOAS) can be used to achieve online monitoring of polluted gas. To improve the monitoring accuracy, the Fourier Transform Filtering Method (FFT) is usually used to process differential absorption spectral data. However, due to the limitation of frequency resolution, its amplitude accuracy is affected, which leads to a large inversion error of gas concentration. A spectral data processing method that combining FFT and FT (FFT+FT) is proposed. First, the panoramic spectrum of differential absorption spectrum data is obtained by FFT filtering, then the spectrum near the peak point is refined by using the improved continuous FT, which can improve the resolution of the characteristic absorption frequency band, the amplitude error is compensated, and the accuracy of gas concentration on-line monitoring is improved. Many SO₂ and NO₂ gases with different concentrations were measured. When the zoom multiple is 15, the maximum measurement error of SO₂ and NO₂ gas is not greater than 3.68% and 3.17%. Compared with the FFT method, the average error decreases by 1.82% and 1.45%, respectively. Compared with the traditional polynomial fitting method, the average error is reduced by 14.9% and 1.80%, respectively. SO₂ and NO₂ gas with the same concentration were measured many times, which verified the stability of the algorithm. The influence of the zoom multiple on the measurement accuracy is analyzed. When the zoom multiple is less than 15, the concentration inversion error decreases with the increase of the zoom multiple. When the refinement multiple increases from 15 to 20, the error increases gradually. When the refining multiple is greater than 20, the errors fluctuate, and all of them are greater than the measurement errors when the multiple is 15. This is because the spectrum lines are too dense due to the large refinement multiples, and the probability of finding the maximum value of the spectrum sequence is reduced. Therefore, when the spectrum with noise is corrected by this method, the measurement accuracy will decrease with the increase of zoom multiple. To reduce the calculation amount of frequency spectrum zooming, the optimal zoom multiple is determined, which satisfies the requirements of measurement accuracy and meets the requirements of DOAS method for real-time online monitoring of pollution gas.