Based on absorption principle, the paper calculated parameters of Beer-Lambert law to measure the light length. The paper analyzed Gaussian lineshape, Lorentzian lineshape and Voigt lineshape, and used the Voigt lineshape to fit the spectrum signal. The peak value of the Voigt lineshape, Lorentzian width and error function were also studied. The direct absorption spectroscopy of TDLAS was used to measure the oxygen absorption spectrum getting the fitted peak value. The light length was calculated as 66.55 cm through putting the peak value into the Beer-Lambert function. The value of comparative measurement was 66.04 cm, and the measurement accuracy was 0.78%. So, this method is suitable in light length measurement.