The resonant photoacoustic spectroscopy system tends to be affected by the external environment, while the relative humidity has negative influence on the photoacoustic signal, which could reduce the accuracy of the photoacoustic spectroscopy technique for trace gas detection or aerosol light absorption coefficient measurement directly. A set of photoacoustic spectroscopy measurement system is built based on a homemade U-shaped photoacoustic cell, and its relative humidity can be precisely adjusted. Performance of the photoacoustic system is measured at different relative humidity, and the impact of relative humidity on photoacoustic signal is analyzed. The corrected parameters of the photoacoustic signal are also obtained based on the experimental results, with which the absorption of water molecules at 7172.699 cm-1 wavelength is measured and the effects of relative humidity are quantitatively analyzed. The result provides reference for the application of photoacoustic spectroscopy technology in different relative humidity conditions.