Tunable Diode Laser Absorption Spectroscopy (TDLAS) has become one of the main diagnostic techniques in flow field diagnostics, and it can be used for non-intrusive and in-situ detection. Wavelength modulation spectroscopy (WMS) and direct absorption (DA) are the two main methods for TDLAS sensing, the WMS method is advantageous for applications with small absorbance, high pressure or hostile flow field, and detection sensitivity is 1～2 orders of magnitude higher than direct absorption. In the field of near-infrared wavelength modulation technology, distributed feedback (DFB) semiconductor lasers have become one of the choices for flow field diagnosis, and whether using fitting of harmonic signals (or normalized harmonic signals) or selecting peaks of harmonic signals to invert flow field parameters, the accurate establishment of the absorption model is very important. The precise representation of the laser frequency-time response and intensity-time response is especially important when modeling, In order to solve the problem of establishing the accurate absorption model, this paper presents a complete method for accurately measuring the modulation parameters of lasers. The modulation characteristics of the 1 392 and 1 469 nm lasers used to detect water vapor absorption are measured. The modulation parameters of the distributed feedback laser with modulation amplitude, modulation frequency and operating temperature are studied. According to the modulation parameters obtained by the method, the absorption model was established. The concentration of water vapor in the air was 197% at room temperature, and the concentration measured by the direct absorption method was 199%. The accuracy of the measurement method of the modulation parameters was verified. The research shows that the modulation depth increases linearly with the increase of the modulation amplitude, decreases monotonously with the increase of the modulation frequency, and increases linearly with the increase of the operating temperature. The intensity and frequency of the laser are modulated simultaneously. Phase difference between the intensity and the frequency is not obvious with the change of modulation amplitude. It increases with the increase of the modulation frequency and decreases with the increase of the operating temperature. The normalized first and second harmonic amplitude increase with the increase of the modulation amplitude, decrease with the increase of the modulation frequency, and are not significant differences with the change of the operating temperature. In the field of absorption spectroscopy, wavelength modulation spectroscopy play an important role, the modulation index is closely related to the peak value of the harmonic signal. When applying the wavelength modulation technique, selecting the appropriate modulation parameters is beneficial to obtain a suitable harmonic signal. The optimal modulation index can be obtained by changing the modulation amplitude, modulation frequency and operating temperature. In this paper, the modulation characteristics of near-infrared distributed feedback semiconductor lasers are studied. The method is also applicable to other types of lasers, which is beneficial to the application of absorption spectroscopy in various fields.