In order to solve the problem that the photoacoustic (PA) detection accuracy of dissolved methane (CH₄) gas in oil is affected by water vapor, molecular relaxation analysis and experimental verification were adopted. The influence mechanism of molecular relaxation effects on PA gas detection was analyzed to explain the reason why environment humidity affects the CH₄ detection accuracy. A distributed feedback (DFB) laser with the wavelength of 1650.9 nm was used to build CH₄ gas detection systems based on a non-resonant PA cell with small volume. The synergistic effect of humidity and operation frequency of PA system on CH₄ detection was verified by the experiments. A high-precision detection scheme was proposed to suppress the interference of water vapor changes on CH₄ detection. PA signals of CH₄ gas at different humidity and frequency were obtained. The experimental results show that the error of PA signal from low frequency to high frequency increases by 10% for dry methane gas. With the increase of humidity, the signal attenuation at high frequency drops from 13% to 2%. When the system works at 40 Hz with an integration time of 30 s, the detection limit of CH₄ gas can reach 0.1×10^?6. This work is of great significance to improve the online monitoring accuracy of dissolved gases in transformer oil.