A water vapor concentration measurement method was studied based on direct absorption spectroscopy techniques. Using DFB diode laser as light source, the concentration of water vapor is obtained through the integrated absorbance inversion. First a low-order polynomial baseline fitting was carried out on sub-regions of the transmitted signal to eliminate the effect of diode-laser output power fluctuation and the absorbance curve was obtained. Then integrated absorbance can be inverted through line-shape fitting using Levenberg-Marquardt non-linear least-squares fitting method. Two absorption lines for concentration measurement near 1.4 μm of water vapor were selected to verify the method.