Light-emitting diode was used as illuminant in the present paper. The light-emitting diode was chosen according to the high absorption in the range of absorption spectrum of NO2. The pollutant concentration can be calculated by measuring the absorption spectrum, and fitting the experimental absorption spectrum and normal absorption spectrum by means of least square fitting method. The method of measuring nitrogen dioxide concentration making use of absorption spectra was studied, the basic principle was analyzed, the process of data analysis was studied including the fitting of experimental spectrum and standard spectrum, wiping off the slow change, using least-square fitting in reverse calculating of concentration etc. The detecting precision was improved by the method of absorption spectrum. In this experiment the light emitting diode was used as illuminant, which made the detecting become not only convenient but also fast, and online real time supervising can be realized. Taking advantage of the characteristic that NO2 has strong absorption in the range of 300-500nm, the NO2 concentration of sample was measured.