Abstract: Based on the monitoring data from 1986 to 2002,it is found that Lishui City is suffering heavily from acid precipitation and the situation tends to become more and more serious.The acid precipitation is characterized by high frequency of occurrence in spring and winter,low pH value,and decreasing ratio of SO₄（2-SO₄^2-/ NO₃^-,which indicates that the source of air pollution is changing from coal burning to a mixture of coal burning and automobile emission.Coal consumption rate,topographic features,frequency of the appearance of low altitude inversion layers and frequency of surface weak winds are found to have significant effects on acid precipitation and pH value of the rain.

Key words: acid precipitation, small basin in mountainous area, cause of formation； Lishui City