Acid rain is a global environmental issue and causes serious adverse impacts on natural ecosystems and human health. The acid rain and its subsequent impacts have been extensively studied in temperate regions and in big cities globally, but the monitoring of acid rain in non-urban areas in subtropical region is still limited. Here, we analyze 18 years of water acidity observations from a non-urban site in western Pearl River Delta region, south China. From 2000 to 2018, annual mean precipitation pH values in the study site showed a marked increase (P<0.0001) from 4.96 in 2000 to 6.88 in 2018. This 18-year dataset of precipitation acidity shows a clear recovery from acid deposition over the annual period, dry seasons and wet seasons. Within a year, precipitation pH was relatively lower in dry seasons (5.46±0.95) than in wet seasons (5.80±0.89) during the whole study period, and the seasonal pattern of precipitation pH was amplified during drier years as a strong dilution effect was detected between precipitation amount and its acidity. The pH of surface runoff water (4.74±0.70) was significantly lower than that of precipitation as a result of the regulating effects of the highly acidified soil and the rich humus on the forest floor. The groundwater pH (6.32±0.63) was significantly higher than the precipitation pH especially during dry seasons. The results of the present study indicated that the groundwater has a strong acid buffer and neutralizing effects and thus plays an important role in supplies of clean water. Long-term changes of water acidity in an intact forested watershed in south China