In the present paper, the infrared spectrum of aerosol and NH4NO3 was analysed and compared, and the result showed that the infrared spectral features of aerosol are consistent with that of NH4NO3. The absorption coefficient α and the mass absorption cross section κ of NO-3 was calculated by the transmissivity of NO-3 in NH4NO3 according to Beer-Lambert law. Then the imaginary part of the complex refractive index was calculated through α. The real part of the complex refractive index was derived from the K-K（Kramers-Kroning）relationship. It has important significance for further research on the scattering and absorption characteristics of the different composition of the atmospheric aerosol through analysing the results of the experiment.