The two-line method is usually used to measure the temperature and mole fraction of gases simultaneously in tunable diode laser absorption spectroscopy. However, in practical applications of the two-line method, there exists some limitations such as cross-talk in the frequency division multiplexing method or reducing temporal resolution in the time division multiplexing method. To solve this problem, a new calibration-free one-line method based on wavelength modulation spectroscopy is proposed to implement simultaneous measurement of gas temperature and mole fraction by using a single absorption line. Using main combustion product CO2 as the target gas, and selecting R(50) spectrum line of CO2 at central frequency of 5007.787 cm -1, we verify the calibration-free one-line method. The temperature is extracted from line shape of the background subtracted peak normalized second harmonic signal, and mole faction of CO2 is extracted from the background subtracted first harmonic normalized second harmonic signal. The experimental results show that the maximum relative error between temperature from the calibration-free one-line method and measured temperature is less than 2.5%, and the maximum relative error of mole fraction is less than 2.8%. It is feasible to measure the temperature and mole fraction of CO2 at the same time using the calibration-free one-line method.