In the long-term operation of distributed feedback laser, there exists a problem of wavenumber drift of the output laser center; this affects the measurement accuracy of gas concentration by fixed-point wavelength modulation spectroscopy. In order to solve this problem, a calibration-free fixed-point wavelength modulation spectroscopy based on the wavenumber drift-correction algorithm is proposed for the calibration-free measurement of gas concentration. The absorption spectrum at 4958.97 cm -1 is selected to measure the concentration of CO2 gas for validating the accuracy of this method. The experimental results show that the proposed correction algorithm overcomes the adverse effect of wavenumber drift of the output laser center on the measurement, and effectively improves the accuracy of gas-concentration measurement.