Studies indicate that current composition of atmospheric aerosol is complex, and its water-soluble ions affect air quality. The ions can produce significant negative effect on the human health. Therefore, rapid and reliable detection of water-soluble ion is an important research work in environmental science. Take SO2－4 ion as an example, the extinction coefficient of water-soluble aerosol is measured by Fourier transform infrared spectroscopy (FTIR) technology, and the complex refractive index N is also calculated through theoretical analysis. By measuring the mass extinction cross section k and extinction coefficient α of the atmospheric aerosol water-soluble ion, the imaginary part ni of the complex refractive index N is calculated according to the extinction coefficient α and the real part nr of the complex refractive index N is also calculated by using the KK (Kramers-Kronig) relationship. The experimental results are also analyzed and discussed.