Tunable diode laser absorption spectroscopy (TDLAS) has been widely used in the gas concentration and temperature online monitoring because of its excellent characteristics such as fast response, high spectral resolution and non-contact feature. In order to reduce error values and simplify experimental equipments, the temperature inversion theory and derivation formula based on wavelength modulation absorption spectroscopy are analyzed. Two distributed feed back (DFB) lasers with output wavelengths of 760.21 nm and 760.88 nm generate laser covering two oxygen absorption lines with time-sharing sawtooth scanning, respectively. Only one laser beam passes through the same optical path after coupling by a 2×1 fiber coupler. Average temperature in the tube-furnace is obtained by the peak-ratio of second harmonic signals. A method to calibrate required parameters for temperature inversion at indoor temperature is used to correct the impact of line-shape and spectral intensity. The experimental results show that the measured temperatures and the preset values within the temperature of 300 K~900 K are consistent, and the measurement error is within ±20 K.