For absorption linewidth inversion with wavelength modulation spectroscopy (WMS),an optimized WMS spectral line fitting method was demonstrated to infer absorption linewidth effectively,and the analytical expressions for relationships between Lorentzian linewidth and the separations of first harmonic peak-to-valley and second harmonic zero-crossing were deduced.The transition of CO2 centered at 4991.25 cm-1 was used to verify the optimized spectral fitting method and the analytical expressions.Results showed that the optimized spectra fitting method was able to infer absorption accurately and compute more than 10 times faster than the commonly used numerical fitting procedure.The second harmonic zero-crossing separation method calculated an even 6 orders faster than the spectra fitting without losing any accuracy for Lorentzian dominated cases.Additionally,lincwidth calculated through second harmonic zero-crossing was preferred for much smaller error than the first harmonic peak-to-valley separation method.The presented analytical expressions can also be used in on-line optical sensing applications,electron paramagnetic resonance,and further theoretical characterization of absorption lineshape.