Through the climatic analysis of fog during 1980～1996，the weather situation was summed up as weak high pressure and cyclone inverted trough over Yancheng city in Jiangsu province.Base on the indexes,the local meteorological elements were analyzed.The diagnose for five indexes of fog occurrence was conducted by the use of probability addition principle.The fog forecasting system was built with the aid of the meteorological information comprehensive process system,and the better effects in operational practice were achieved.