Abstract: Using the observation data of 26 acid rain stations in the Beijing-Tianjin-Hebei region from 2012 to 2017,the spatial distribution characteristics of precipitation pH value and acid rain frequency were studied based on the GIS interpolation method.The variation characteristics of acid rain in different months,seasons and at different precipitation grades in the acid rain area were analyzed.The causes of acid rain formation were studied.The results show that from 2012 to 2017,the area of acid rain influencing in this region decreases.The areas with severe acid rain from 2012 to 2014 and with a high frequency of acid rain from 2012 to 2013 are both mainly distributed in the junction region of Chengde,Tangshan and Qinhuangdao cities.Compared with 2012,the acid rain area (accounting for 15% of the total region area) and the area with a high frequency of acid rain (accounting for 17% of the total region area) in 2017 decrease 63% and 52%,respectively.Generally,the frequency of relatively weak acid rain is the highest.83% of the extraordinarily strong acid rain occurs in 2012 and 2013.Compared with other seasons,the average pH value of in autumn precipitation is the lowest,its frequency of acid rain occurrence is the highest,and its pollution extent is the most serious.The increase of air pollutants in winter leads to the increase of K value in precipitation.The average pH value in the moderate-graded precipitation is the smallest.For the light-graded precipitation,the ratio of acid rain occurrence times to the total acid rain occurrence times reaches the maximum,and the corresponding K value in precipitation is the largest.After a rainstorm,acid rain pollution is reduced.After 2011,the emission of NO_x exceeds that of SO₂.The acid rain pollution has gradually changed from sulfuric acid-nitric acid type to nitric acid type.Reducing the emission of SO₂ and NO_x is an important measure to reduce the acid rain pollution in the Beijing-Tianjin-Hebei region.