Tunable diode laser absorption spectroscopy (TDLAS) technique is a non-intrusive measurement method for measuring gas concentration and temperature,and it requires scanning over the entire spectrum. To relieve the overlapping spectral interference and increase the measurement rate, a fixed-wavelength absorption spectroscopy technique is used. To study the effect of fixed-wavelength absorption spectroscopy for temperature measurement, a distributed feedback laser with a center wavelength around 2.0 μm is used to measure the temperature of CO₂. Since the absorption spectrum peak is affected by the lineshape function, the peak value of the lineshape function is simulated by querying the database, and then the relationship between the absorption peak ratio and temperature under the test condition is simulated. Finally, a tube furnace is used in the validation experiment. The experimental results show that the measured temperature is highly consistent with the set temperature. In the range of 673-1273 K, the average temperature measurement error is 1.22%, which proves the feasibility and accuracy of the fixed-wavelength absorption spectroscopy technique.