Water vapor is a key parameter in climate change and quantitative remote sensing. For ground based sun-photometer data，the modified Langley method can be used to derive precipitable water (PW). In this paper, the authors retrieved PW by means of 940 nm water vapor high-absorption channel and non-gas absorption 870 nm and 1 020 nm channels. After deriving aerosol optical thickness (AOT) of 870 nm and 1 020 nm channels, AOT of 936 nm was interpolated by using the Angstrom law. Using simulation with the MODTRAN 4.0 radiation transform model under different atmospheric models and zenith angles，the authors detected the relationship between PW and atmospheric transmittance. Then PW was retrieved every two minutes by means of the modified Langley method. The results show that the temporal variation of PW in autumn remains remarkable in the study area，and the main error source seems to be the cloud effect. This method is useful in atmospheric and thermal remote sensing studies.