The air flow and turbulent fluxes features during the radiation fog formed on the dawn of 17 October 2007 is discussed in order to study the mechanism of an unexpected night fog, based on the meteorological and turbulent data obtained from the 250 m height tower in Tianjin, as well as the NCEP reanalysis data and other observational data. The results show that the lower layer easterly flow coming from the south region of the Northeast cold high pressure led to remarkable temperature fall and humidity value increase in the daytime prior to the fog formation, which quickly turned the dry boundary layer to be moist. The vapor transfer indicated that the vapor of the radiation fog was provided by the easterly advection from Bohai Sea, not from local area. Turbulent vapor fluxes increased ten times as that before the fog. The horizontal vapor fluxes transported against the wind direction, which led to the escape of water vapor from Tianjin city and the dissipation of fog.