Abstract: Based on conventional weather maps, automatic weather station observations, radiosonde stations in upper-air data, the author analyzed the persistent fog which happened in 7th to 9th, January, 2009 in northern Anhui, to investigate the cause of the fog. The results showed that: the fog type was advection fog, shifted to the warm and humid air from the southwest encountered the cold advection under the control of the underlying surface, the rapid cooling of air caused condensation generated; intersection of cold and warm advection was the formation of stratospheric on the basis of the fog, the larger temperature difference between a large number of slow-moving warm air and cold advection control the underlying surface was the objective conditions for the formation of advection fog; near surface layer 1-5 m/s the southwest wind or westerly wind, a steady stream transport of water vapor, was conducive to the generation of fog and continuous; 925 hpa below the level, there were 1-2℃ temperature inversion, could stop up the water vapor layer, the formation of fog was not easy to dissipate.