This paper presents an analysis of variation characteristics and long term trend of acid rain in Taishan Mountain area, using the observed data of Taishan Mountain Weather Station from 1992 to 2010 and the data of Tai’an Weather Station from 2007 to 2010. The two stations are located within a distance of 10 km, and the latter sits at the foot of Taishan Mountain. The annual average precipitation pH values of the Taishan Mountain Station are generally below 5.60 except the two values in 1999 and 2001, with a variation ranging from 4.31 to 6.47. These pH values reveal two different periods of variation: one is a fluctuation change with no significant trend during 1992-1999, and another is a steady decrease trend from 2000 to 2010 at the annual rate of -0.15 a-1. The monthly average precipitation pH values over these years at Taishan Mountain Station vary from 4.50 to 5.02 with higher values in March and July and lower values in August and September, having an irregular seasonal variation. The monthly average pH values of Tai’an Weather Station have similar features in seasonal variation with Taishan Mountain Station, except the irregular changes in a few months due to limited data. Compared with the acid rain data from other mountain weather stations in China, the precipitation acidity of Taishan Mountain Station is much stronger than that of the mountains in the Northwest of China, similar with that of mountains in the Southwest of China, but a little weaker than that of Dayao Mountain, which is located in Guangdong Province, South China. The comparison of precipitation pH values and NHCs between Taishan Mountain Station and Tai’an Station indicates that the pH values and average NHCs of Tai’an Station are higher than those of Taishan Mountain Station. The most ΔpH values and ΔNHCs, which means the differences of pH values and of NHCs between Tai’an Station and Taishan Mountain Station when the two sites see rains at the same time and it begins to rain in less than two hours, are positive. This is probably due to the different contributions of scavenging the below clouds of atmospheric pollutants like aerosols， SO2 and NOx to the chemical composition of the rainwater at the two stations.