The gas concentration calculated from second harmonic signal would be affected by the variety of linewidth induced by different gas parameters during the measurement by wavelength modulation absorption spectrometry using tunable diode laser. The effect of linewidth variety on second harmonic signal for Lorentz lineshape was analyzed. A correction method using ratio of peak to valley from second harmonic signal was also given here. The experiment on CO2 concentration at 1578.22 nm was carried out in a 10 cm absorption cell with different modulation amplitudes, gas pressures and concentrations to validate the effect of linewidth variey on second harmonic signal. The results show that the relative errors of gas concentration would be decreased from 34.3% to 1.8% by correction method when the pressure rises from 1.0×105 Pa to 1.5×105 Pa, and decreased from 12.8% to 1.8% by correction method when the gas concentration changes from 20% to 100%. The correction method can eliminate the effect of linewidth variety on gas measurement by second harmonic signal effectively.