Based on the in site investigation data obtained during the period from June 2001 to May 2002 and the history data from Liyang Environmental Protection Bureau in Tianmuhu Lake, the evolution trend of water environment is analyzed in detail. The results show that water quality decreases significantly and continuously aggravates with fishery and increase of tourists. The main water environment problem is water quality in Tianmuhu Lake has evidently been deteriorated. The symptom of the water deterioration can be seen from the fact that blue-green, and green algae quickly reproduce and sight quality decreases. Having done some field monitoring and over analysis of the trend of water environment in Tianmuhu Lake, we found that the main causes are: the increases of aquiculture and accommodating facilities in resort. Aiming to the present situation of water environment, the countermeasures of protecting water reserve and ameliorating water quality are given: (1) Strictly controlling the sources of pollution; (2) Restoring macrophyte; (3) Adjusting framework of aquiculture; (4) Restoring vegetation and controlling the input of nitrogen and phosphorus; (5) Intensifying the management of valley; (6) Enforcing ecological tourism.