Incoherent broad-band cavity-enhanced absorption spectroscopy (IBBCEAS) is gradually becoming one of the primary methods for measuring NO3 radical with the advantages of high selectivity, high sensitivity and high spatial resolution. However, due to the limited spectral resolution of the adopted spectrometers, it is not enough to distinguish the fine absorption structures of water vapor, which results in the non-linear absorption of water vapor and thus affects the accurate retrieval of NO3 radical concentration. A method based on interpolation for obtaining the effective cross section of water vapor absorption is introduced, which is used for the elimination of the interference of water vapor absorption on the concentration retrieval of NO3 radical in the IBBCEAS device. The water vapor spectra under different water concentrations are measured to obtain the effective water vapor absorption cross section by the interpolation method. The effective water vapor absorption cross section is used to retrieve water vapors with different water concentrations. The linear correlation coefficient between the retrieval results and the data from the commercial hygrometers is 0.99789. On this basis, the absorption spectra of NO3 radical and NO2 gas with different water vapor concentrations are measured and fitted. There is no water vapor absorption structure in the residual spectra, and the linear correlation coefficient between the retrieved water concentrations and the measurement values from the commercial hygrometers is 0.999. The detection limits of NO3 radical and NO2 within an integration time of 30 s are 5.8×10 -12 and 3.6×10 -9, respectively. This system is applied to measure the concentrations of NO3 radical and NO2 in the atmosphere at night, the measured volume fraction of NO3 radical is 18.4×10 -12-22.9×10 -12 with an average volume fraction of 20.2×10 -12, while the measured volume fraction of NO2 is 0.6×10 -9 -16.0×10 -9 with an average volume fraction of 9.9×10 -9. The experimental results show that the effective water vapor absorption cross section obtained by the interpolation method can be used to effectively eliminate the effect of water vapor absorption on the retrieval of NO3 absorption and to improve the accuracy of NO3 radical and NO2 gas concentration measurement.