Fog is a severe weather hazard that greatly influences traffic and daily life with potentially heavy economic loss. Based on the 503 observed fog station data from 1958 to 2007, the temporal and spatial characteristics of fog days in autumn and winter were analyzed. It was found that the fog in autumn and winter occurs frequently over Southwest China. The annual average number of fog days in autumn and winter in Southwest China is more than 18 days, which is twice as many as that in eastern China in the same period. Moreover, there is a significant climate variability of fog days in autumn and winter in Southwest China, which is mainly reflected in the interannual and interdecadal scales, and there are significant differences in the meteorological conditions of fog days in different climate scales. On the interannual scale, the north wind anomaly in the middle and upper levels is more significant, which brings the cold air from the north to the southwest, causing the cold air anomaly in the middle and upper levels over the southwest. Moreover, due to the strong sinking movement over the region, the cold air in the upper levels is brought to the lower levels. At this time, because of the abnormally high temperature and humidity in the lower levels, the cold and warm air converges over Southwest China which is located in the north of the rain belt. In a result, the air is easily oversaturated, resulting the number of fog days increasing. On the interdecadal scale, the anomalous northerly wind in the lower level is more significant compared with the interannual scale, which brings the cold air in the lower level from the north to the southwest. This results in the decreasing of temperature, specific humidity and temperature dew point difference and further leads to the supersaturation of water vapor in the Southwest China because the temperature drops faster than the humidity. At the same time, the atmosphere is relatively stable, increasing the number of fog days.