Abstract: The climatic characteristics and background were analysed by using the observational data of fog in Chengdu in the period of 1981 ~ 2010. The results showed the fog days had a decreasing trend by 3.2 days per year. The change was obviously different in each season. The fog days had a significant change which the formation was mainly concentrated in 5 to 8 o'clock and the dissipation was in 8 to 12 o'clock. The spatial distribution showed the fog was too much in south,and was little in northwest and east of Chengdu. The decreasing amplitude in the northwest and eastern is less than the south. The fog occurred with the highest frequency when the pressure was 950. 0 to 970. 0 h Pa,the temperature was 5 to 15 degree,the relative humidity was 70 to 90,the wind speed was 0 to 3 meter per second and the inversion was in surface.