Absorption of water vapor distributes in the whole mid-infrared bands, and it interferes with the quantitative analysis of infrared spectra. Impact of spectral absorption of water vapor on measurement precision is analyzed. The gas without dryness and after dryness are measured by Fourier transform infrared spectroscopy(FTIR), and the precision and detection limits of CO, CO2 , N2 O and CH4 are compared in the different measurements. The results show that the precision and detection limits were improved after drying of the sample gas.