The principle of differential optical absorption spectroscopy (DOAS) for measuring the concentration of air pollutants is presented. The adjustment of measured spectra and the least-squares method are also described .We have been using this method in retrieving the concentration of air pollutants and measuring the air pollutants for a long time and the results were coincident with circumstances. This method was proved very suitable for retrieving the concentration of air pollutants in DOAS system and provides a good technique for environmental monitoring.