The gas temperature measurement was achieved by using wavelength-modulation spectroscopy technique based on the Tunable Diode Laser Absorption Spectroscopy (TDLAS). Considering that H2O exists in the atmosphere and industrial production, we choose H2O as the carrier gas of temperature measurements. Using the absorption lines near 1 397 nm, according to analysis the relationship between the second harmonic signal peak - peak ratio and temperature, temperature measurements were obtained. Experimental results show that the max error does not exceed 0.113% within the temperature range of 400 ~1 000℃. Combined with the special structure, the device can be used for temperature monitoring of industrial production process. The range of temperature monitoring instrument, the monitoring precision and accuracy is far better than the currently used. The temperature monitoring can provide accurate data for the production equipment of boiler combustion, so as to realize the production efficiency and accurate control, and achieve the ultimate goal of energy saving and emission reduction.