Abstract: The most important variable parameter of a roof rainwater harvesting system is the tank volume and the optimized factors for the tank volume including rainfall, roof catchment area, number of residents and water consumption, etc. An optimization model to determine the tank size of the roof rainwater harvesting system was constructed by the linear programming method. With the maximum variable economic benefits in the life cycle of the system as the objective function, the constraint conditions were determined and the data of rainfall in Nanchang was simulated by the model. The calculation results show that excessive low water price without market regulation could make it difficult to acquire economic benefits; either the roof catchment area of buildings or monthly water consumption was the limit factor to the tank size, and usually the roof catchment area is the limit factor for residential buildings; the roof rainwater harvesting systems in high-rise buildings or commercial buildings could acquire more financial benefits.