A new fast gas recognition algorithm was developed based on passive Fourier transform infrared spectrometer, and then the improved momentum gradient descent method was used to realize the fast fitting the measured brightness temperature spectra. This method does not need to measure the background spectrum in advance, and can directly subtract the background interference of atmospheric gas and sky from the measured spectrum. Besides extracting the composition and concentration of the pollutant gas, it can also obtain the concentration-path-length of the main gases in the atmosphere in real time. This method is suitable for the identification and analysis of the pollutant gas with weak signals in the low altitude background.