The principle of a differential optical absorption spectrometer (DOAS) is presented in this paper. UV-VIS light that emits from a high-pressure arc lamp is collimated by the telescope, reflected by the retroreflector, collected by the same telescope. In the transmission, different gases have different absorptions in different wavebands which can distinguish the different gases, and the spectra have carried the characteristics of air pollutants, so we can retrieve the concentration of air pollutants by comparing the measuring spectra and the lamp spectra. We use this method to measure the organic pollutants. In processing and analyzing the spectra, we use some mathematics algorithm, such as the polynomial of specified degree, least squares fitting algorithm, etc, so the concentration can retrieve precisely. In this paper, we use DOAS method to measure the spectra of benzene and toluene sample and analyze the results. The results anastomose with the theory. The method will be the reliable one for measuring and monitoring the atmosphere organic pollutants.