The differential optical absorption spectroscopy (DOAS) has become a widely used method to measure the concentration of trace gases in the atmosphere. By using the least squares fits of reference spectra to the measurement spectra, it usually has a high measurement precision. But in order to determine the detection limit, the error must be considered seriously. The measurement error in DOAS is analyzed, it is caused mainly by noise, selection of waveband and interferences of other trace gases. The relationship between error and those factors is discussed.