Based on the data of fog from 9 weather stations in the central coastal plain of Hebei Province during 1971—2000.The formative characteristics and it’s decadal change of fog are analyzed.The results show:1)Fog is one of disastrous weathers of highest frequency in winter and autumn in the central seaside plain;2)In addition to low visibility,continuous occurence,long duration and large range are also the formative characteristics of fogs,which have disastrous impacts,and can not be neglected;3)After the 1990s,the annual number of heavy fog day has evidently increased in comparision with the 1980s and 1990s;4)The main reason for increase in fog days is the changes in weather and climate condition,and it is also closely related to human activities which lead to the development of city economy and the excerbation of air pollution.