Tunable Semiconductor Laser Spectroscopy (TDLAS) has been a rapidly developing spectral detection technology in recent years. Compared with other spectral detection technologies, TDLAS has the advantages of high sensitivity, high resolution, real-time monitoring, good portability and miniaturization, and has been widely used in the fields of industrial environmental protection, medical detection, meteorological monitoring and so on. The harmonic signal in the TDLAS wavelength modulation method is susceptible to air pressure. It is found that the influence of air pressure is the influence of modulation depth on the harmonic signal. Based on the principle of the harmonic method in TDLAS technology, the relationship between each harmonic and modulation depth is studied. The modulation depth of the current air pressure environment is calculated by calculating the center amplitude ratio of the fourth harmonic to the second harmonic. To adjust the amplitude of the modulation frequency so that the modulation depth is close to the optimal modulation depth value of each subharmonic, and the signal-to-noise ratio of the harmonic signal is optimal to improve the detection accuracy. The second and fourth harmonic signals under 10.2~177.9 kPa pressure were extracted by the TDLAS water vapor detection system. The simulation and experimental analysis were carried out. The simulation results show that the maximum relative error of the center amplitude ratio of the fourth harmonic to the second harmonic is -1.44%, and the maximum relative error of the modulation depth simulation to the theoretical value is 1.78%. The experimental results show that the modulation depth value is calculated based on the modulation depth function. When m=2.226 7, the measured central frequency amplitude of the second harmonic reaches the maximum value. When m=4.061 0, the measured central frequency amplitude of the fourth harmonic reaches the maximum value, which is consistent with the theoretical results. When 30.2 kPa<p<177.9 kPa, the relative error between the modulation depth and the pressure product MP value is small. The maximum relative error is not more than ±3.2%. It shows that the MP value under this pressure does not fluctuate much. The modulation depth value calculated by the modulation depth function approximates the actual value, which verifies the accuracy of the modulation depth function theory.