In this paper, we present a method of measuring atmospheric water vapor concentration by using infrared differential optical absorption spectroscopy (DOAS). The experimental setup is converted from a self-made non-dispersive infrared multi-gas analyzer. In the process of DOAS retrieval, the reference absorption cross section is calculated by applying the Voigt broadening method to the absorption lines from HITRAN database. The influences of temperature, pressure and instrument function are also taken into account in the calculation. A validation study of the water vapor measurement is performed by comparing the results measured by a non-dispersive infrared analyzer. The results show good agreement with each other (correlation coefficient = 0.93347). It indicates that the infrared DOAS technique has the potential applications to other gases measurements which have no or weak absorption within the UV region, e.g. CO2, CH4, CO, N2O, etc.