The acid rain data during “Eleventh Five Year” period was used to analyse the trend, characteristics and reasons of acid rain in Shanghai. The results showed that the low value area of pH and high value area of acid rain frequency expanded significantly. The changes of the chemical composition confirm the effectiveness of the measures for desulphurization and dust control in Shanghai. But the precipitation acidification of Shanghai continues. The main reason is that the relative change of the strong alkaline calcium and the weak alkaline ammonium causes that alkaline ions can’t completely neutralize the sulfate which had the highest ratio and the increasing nitrate in precipitation.