Based on the historical observation data of acid rain at Baoshan and Pudong in Shanghai,the long-term trend and formation of regional acid rain changes and the spatial distribution of precipitation acidity were analyzed.Generally speaking,precipitation at Baoshan site shows weak acidity,the mean rainfall pH is 5.21 and the mean acid rain rate is 24%.The inter-annual variation of rainfall pH shows a slightly downward trend.The electrical conductivity K increases obviously year by year with an average annual increase at 2 μs·cm^-1.The acid rain rate fluctuates from 8% in 2004 to 48% in 2009.The rainfall pH is usually lower in cold season and higher in warm season.It is the opposite for the electrical conductivity K.The average pH of acid rain in Pudong is 4.38,lower than that in Baoshan at 5.08.The acid rain rate in Pudong is 79.5%,much higher than that in Baoshan at 33%.Atmospheric pollutants SO₂,NO₂ and PM₁₀ concentrations are negatively correlated with rain pH and positively related to electrical conductivity.Atmospheric particulate pollutants,chemical composition,precipitation and wind direction exert important effects on the changes of precipitation pH and acid rain rate.