In this study, a gas volume fraction-inversion method is proposed to solve the drifting issue of the second harmonic background signal in the wavelength modulation spectroscopy technology. Here, the signal similar to the measured second harmonic signal in the background signal history data is taken as the background signal. First, a laser with a central wavelength of ~5.18 μm is used to build a NO volume fraction measurement system and a background signal library is constructed using the pure N2 background signal measured within 48 h as historical data. Subsequently, the NO is passed into the system, and the absorbed second harmonic signal and the correlation coefficient between it and each background signal in the background signal library is calculated. Finally, the volume fraction of the gas is retrieved after subtracting the background signal from the maximum correlation coefficient. The NO with a volume fraction of 2.5 × 10^-6 is monitored for 24 h, and the results show that the method could decrease the average relative error from 6.48% before correction to 3.84%.