The differential optical absorption spectroscopy (DOAS) has become a widely used method to measure the concentration of trace gases in the atmosphere. The key to retrieving the concentration of trace gases is to get the spectrum with the fast and accurately in the DOAS system. The design and performance analysis of optomechanical scanning spectrometer is presented in this paper. It has been shown that it satisfies the demand of air quality monitoring system from the shape of calibration spectrum line and the result of measuring SO2 sample.