The calibration-free wavelength modulation absorption spectrum is studied based on the second harmonic (2f) signal to remove the calibration procedure for tunable diode laser absorption gas sensing. The simulated 2f signal is obtain from the analysis of peak and trough from measured 2f signal line shape. The gas concentration is calculated by the linear fitting of measured and simulated 2f signals. The experiment on CO2 detection is carried out at transition of 6336.24cm-1 in a 10 cm absorption cell. The results prove that the calibration-free wavelength modulation absorption spectrum is suited for on-site gas sensing at various conditions. The average absolute error of gas concentration is 0.67% at modulation index ranging from 1.8 to 3.2. With gas concentration and pressure varied, the average absolute errors of gas concentration are measured to be about 0.98% and 0.74%.