Aiming at improving the measurement of wavelength modulation via laser absorption spectroscopy, a non-dispersive multi-wavelength second-harmonic measurement method is proposed. By adjusting the relative position between the spectral lines, the demodulation of the multispectral line modulation signal can be achieved without the need for a grating or other spectroscopic devices, and the gas temperature can then be measured online. Based on the 7185.60 cm^-1/7444.35 cm^-1 H₂O absorption spectrum, the influence of the modulation coefficient and the relative position between the spectral lines on the measurement are discussed. Further, the second-harmonic extraction method under different modulation frequency conditions is analyzed using the modulation signal spectrogram. A numerical simulation method is used to establish a complex flow-field change model in the pulse detonation process. The proposed method is used to measure gas temperature during the pulse detonation process, and its correctness is verified in this study. The research results are of great significance to the measurement of wavelength modulation under complex environmental conditions.