The project of Water Diversion from Yangtze River to Lake Taihu has played an important role in preventing flood, and brings about great integrated benefits on society, economy and environment in the Taihu Basin since 2002. The paper discusses the methodology for evaluating the integrated benefit of Water Diversion, and uses analytical hierarchy process and weighted synthetic index to determine the integrated benefits of the project. Moreover, the paper uses market pricing method to determine the direct benefit of saving chemicals in the water works within the region. The value of synthetic index of integrated benefit is U=77, that means the project has large benefits to economy, society and environment of Yang-Cheng-Dian-Mao District, especially the water quality and quantity of rivers in the Taihu Basin has improved to meet water source quality, reducing health risk, promoting integrated utilization of water resource, improving living environment, expanding aquiculture benefit, saving water works expenditure, gaining industry water supply and agriculture benefits, etc. During 2002-2005, the four year benefit from improving drinking water quality by the project was 9.9 million, 29.2 million, 36.2 million, 35.1 million respectively, and totally 110.4 million. According to the proportion of economic benefit with integrated benefit, the imputed economic benefit was about 4.8 billion; consequently the integrated benefit was about 5.3 billion.