Abstract: To quantitatively calculate the amount of subsidy is very important for the government, since cash incentive is an effective method to promote rainwater collection. In this study, we investigated the volume of collected water from the roof in different rainfall conditions and the amount of consumed water to wash cars in the representative climatic situations through field tests. Then we assessed the cost and benefit of the project. The calculation results show that the project could offer such a low net income ratio as to be encouraged and supported by the government although the ecological and social benefit was obvious. The average net benefit rate in 10 years was lower than those of other water conservancy projects, for it was only 7.05 %, and was so far as to 3% as the inflation rate 4% was subtracted. Combining with socio-economic development status in Dalian, this paper also presents several incentive measures, in which the amount of cash subsidy is counted according to the quantitatively calculated eco-benefit. Compared to other means, the method mentioned above is more reasonable.