Differential optical absorption spectrometry(DOAS) has been widely used for various polluting gas concentration measurement,which influence the measurement precision of the main factor is gas absorption section measurement.This paper using LambertBeer absorption law and the design of my own measuring device measuring the Absorption Cross Section of NO,and used polynomial fitting method to remove the Rayleigh scattering,Mie scattering and molecular absorption,including broadband slowly varying spectrum in the Differential absorption spectrum of NO,improves the accuracy of measurement.Then using leastsquare method inversion gas concentrations.The author inverted gas chroma value of different concentrations NO according to the measured absorption section,and achieved good effect.But this method have not obtained very ideal effect in the low concentrations,therefore,the author next work is to improve the measuring precision of the absorption cross section and inversion precision of the gas absorption cross section.