There is water vapor absorption near 940 nm band, so the water vapor abundance can be retrieved on the basis of its functional dependence on the transmission in the 940 nm band. The method for determining water vapor abundance by using sunphotometers is described in detail. The clear-sky data of Hefei from 2005 to 2006 are analyzed. The results show that in Hefei the vertical water vapor abundance in spring and summer is higher than that in autumn and winter. In July and March its daily relative standard deviations are 2.34% and 6.27% respectively, so the water vapor abundance daily variation is small in clear days.