Abstract: This paper presents a rainwater storage power generating system comprised of a split-flow and purification device and an impounding and generating unit, which is designed to make full use of the potential energy of rainwater collected on building roof. Designed by the siphon principle, it is capable of impounding rainwater in a drainage pipeline, then releasing it to generate power. In analysis of this system, Bernoulli equation is applied to deriving the analytical relationships of its instantaneous velocity, flux, power and time versus water head, and the design is optimized using its instantaneous peak power as the objective function and its peak vacuum as the constraint. We optimize and analyze the geometrical parameters of its siphon η-tube, select 19 sets of tube height parameters first, and then determine the best set of parameter combination (4.5, 6.0) via maximizing the power that is generated through one releasing operation of the water into a unit length of the pipeline. We conduct prototype experimental tests, verify that our power generating system meets design specifications, and develop a formula of the system’s power generation during one cycle of rainwater impounding and releasing. The results of this study, particularly the device and experimental facility, are helpful to rainwater use and further studies on rainwater power generation.