Abstract: Differential optical absorption spectroscopy (DOAS) is a widely used method to measure trace gases in the atmosphere. The concentrations of trace gases can be retrieved by fitting differential absorption spectra with standard differential absorption cross-section using the linear least-square method. The basic principle of DOAS is introduced. The construction of DOAS on-line monitoring system was designed and the retrieval method of trace gases concentration based on the principle of least squares was discussed. The properties of DOAS system were tested by experiments. The advantages of DOAS system used in atmosphere quality monitoring are proven.