Abstract: Rainwater harvesting project by agricultural greenhouse could provide reliable water source for urban agriculture around water scarce cities. However, guiding theory of optimal size for water storage pool is absent. It will lower the efficiency and benefit of rainwater harvesting project. To provide water for agriculture and enhance economic feasibility of rainwater harvesting project, principle and methodology for optimizing storage pool was developed in this paper. Research on existing agricultural rainwater harvesting project showed that rainwater harvesting efficiency was influenced by difference of daily rainfall and size of storage pool. In the case of certain rainfall and catchment area, the amount of rainwater harvested increases with increasing of storage pool. But the amount by unit volume of storage pool decreased. Cost-benefit analysis showed that the ratio of benefit to cost of the project was relatively high when design daily rainfall is around 25-40 mm. And rainwater harvested could almost meet water need. A case study showed, in normal year, the actual rainwater harvesting rate was slightly lower than the designed rainwater harvesting rate due to temporal distribution of rainfall and agricultural water use. And the ratio of benefit to cost was far lower than that of the project at optimal scale. Thus properly reduction of size of the storage pool could improve the economy of the project. The methodology can provide theory and practical guidance for agricultural rainwater harvesting project.