Tunable diode-laser absorption spectroscopy techniques have been widely used in many regions, such as environmental monitoring and detection of industrial process, due to their high spectral resolution, high sensitivity, fast time response and nonintrusive character. It's important to obtain absorbance curve and integrated absorbance for gas concentration inversion and line strength calibration in direct-absorption spectroscopy techniques. An approach was taken to process the laser direct absorption spectrum. First a low-order polynomial baseline fitting was carried on sub-regions of the transmitted signal to eliminate the effect of diode-laser output fluctuation and obtain the absorbance curve. Then integrated absorbance can be inverted through line-shape fitting using Levenberg-Marquardt non-linear least-squares fitting method. This approach was verified by experimental processing of absorption lines of water vapor.