A measurement method of atmospheric water vapor column density by passive differential optical absorption spectroscopy (DOAS) technology is developed. The solar spectrum at the top of atmosphere can be considered as the reference spectrum and the total slant column densities of water vapor and O4 can be retrieved simultaneously from the measurement spectrum by DOAS algorithm. To convert the slant column density to the vertical column density, the relationship of air mass factors (AMFs) of water vapor and O4 is analyzed. The vertical column density of water vapor is resolved with the AMF of O4. The saturated absorption problem of water vapor affects the retrieval precision of DOAS, so a numerical simulation method is developed to revise the water vapor density. The water vapor measurement result of DOAS is compared with those obtained with sun photometer and GOME-2 satellite. Good correlation is shown, and thus the feasibility of the method is verified.