Abstract: The variational characteristics,distribution,present situation and causes of acid rain are analyzed based on the monitoring data from 1993 to 2000 in Hunan Province.The results show that the central\|south and southwest of Hunan are the main acid rain regions with pH value less than 5\^0 and the centre of Hunan is the region where the rainfall is subacid.The ionic balance of precipitation is generally kept except for Huaihua and Hongjiang.The rainfall is typically sulphur\|type rain in Hunan.The formation of acid rain is mostly influenced by the local sources as well as other natural factors such as earth,landform,climate and transportation of air pollutants.Authors suggest that social factors such as energy structure and industrial pollutants have the decisive effects on the acid rain formation in Hunan Province.