Based on Jiuhua Mountain meteorological stations at different altitudes and regional automatic weather station data， the comparative analysis was made about climate characteristics of fog in mountainous area and flat area，and fog formation mechanism was analyzed also． The results show that the annual change of fog in mountainous area decreased year by year，but fog in flat area presented slowly increasing trend，the peak time of fog occurrence in mountain area was at about 05: 00，which was one hours earlier than that in the flat area，but the highest frequency of fog occurrence at 08: 00，which was one hours later than that in the flat area． The percentage of fog days occurred when temperature cooling at night was more than or equal to 6 ℃ was 74． 4%，and the percentage of fog days occurred when the daily temperature range was more than or equal to 7 ℃ was 80． 9%． When daily mean wind speed was less than 3 m/s， fog weather would appeared，and on 83． 9% fog days the inversion layer existed near the ground，and fog days was positively related to inversion intensity． The convergence role of the bell mouth topographic made water vapor in the flared bottom area to reach saturation and formed fog，and breeze conditions was advantageous to radiation fog formation，if there was wind field convergence near surface，it  was more conducive to the formation and maintenance of the fog，and forest microclimate effects also contribute to fog formation．