Abstract: Fog is a severe weather hazard that greatly influences traffic and daily life, potentially contributing to heavy economic losses. Based on the observed fog data from 1958 to 2007, using correlation coefficient analysis and composite analysis, the temporal and spatial characteristics of fog days in winter were analyzed. It was found that fog in winter frequently occurred, and the interannual variability in winter fog days was relatively strong, mainly over southwestern China, northern China, and Fujian. According to the distribution patterns of the winter fog days, we defined three relatively independent fog areas. The influence of regional climatic conditions on local fog formation mechanisms is discussed from the aspects of water vapor transport, atmospheric stability, and atmospheric circulation. There was a significant difference in climatic conditions for winter fog formation among the different regions. Winter fog formation over southwestern China was less affected by water vapor transport, while more affected by the atmospheric stability and the southward cold air. Winter fog formation over northern China was affected by water vapor transport; they occurred more frequently when water vapor was transported farther north and convective instability occurred over the middle and lower Yangtze River. Meanwhile, the formation and maintenance of winter fog days over southern China were also found to be influenced by vapor transport; when water vapor was transported farther south, the fog occurred more frequently. In terms of atmospheric circulation, winter fog formation was mainly affected by the deepening high-pressure ridge, east of the Balkhash Lake over southwestern China, weakening of East Asia trough and Siberian High over northern China, and the weakening of the Siberian High over southern China.