It has been measured that the pH value and concentrations of nutrients such as N and P etc. in the rain and runoff at the mouth of Meiliang Bay, Lake Taihu from January 1998 to June 2000. Due to the affection of rainfall, the TIN, PO₄^3- and CODMn are amounts to 10-15%, 2.0% and 4-6% of the total pollutes loads that was enter to the lake in the same period respectively. It was measured also that the total hard ions and alkalinity concentrations at the mouth of Meiliang Bay, Lake Taihu, in the March-June of 1999. According to the analysis with historic dada it has been found that the ratio of total hard ions over to the alkalinity in water of lake was raised with time. This phenomenon could reflect the response of aquatic ecosystem, Lake Taihu, to the deposition of acid rainfall in long term.