Rainwater harvesting system (RHS) is small and decentralized water source that compensates the shortcomings of huge dams. Being simple, low cost and free from ownership controvery, RHS is potential for civil participation. This study established a water source development plate form by integrating rainfall data bank and model bank to provide system performance simulation, financial analysis and spatial exhibition of RHS. Using Taipei City as case study for spatial exhibition, rainfall data was first analyzed, and different design was simulated for optimum design. This study overcomes the limitation of traditional point-based design, and enlarges the scale for civil participation to maximally utilize every drop of rainwater for the sustainable utilization of water resource.