The interferences owing to the defects of the grating and the stray light in the spectrometer of DOAS system cause the extra .spectral structures, which are difficult to lie distinguished from the absorption structures of the flue gas. These structures can not be removed by averaging the spectra, due to their relative stability in certain time periods. The lamp spectra taken before the light going to the polluted flue gas reflects the same instrumental structures as the spectra of the flue gas. Therefore it is very useful to reduce the noise from the optical components by dividing the lamp spectra. It is mentioned that in this case both spectra of the lamp and the flue gas should match to each other exactly in the wavelength. In order to solve these problems, two methods have been presented in this paper. It has been shown that the error of retriving the concen tration have been reduced in the experiment.