The wavelength modulation method in tunable diode laser absorption spectroscopy is widely used in the measurement of gas concentration due to its advantages of adapting to harsh environment and high sensitivity. Compared with the common sine wave modulation, the triangular modulation has higher sensitivity. However, the triangular modulation method is generally combined with the calibration approach to measure gas concentration. In this study, a new calibration-free method based on triangular wave modulation is proposed and a frequency response model of distributed feedback laser is developed. The experimental results illustrate that the relative residual of etalon signal is less than 0.4% and the relative residual of the first harmonic normalized second harmonic signal is less than 1.2%, which prove the feasibility of the proposed model. The calibration-free measurement of CH4 gas concentration based on triangular wave modulation is realized.