In Portland and Seattle, USA, there is plenty of rain all year long. Landscape architects often use rain gardens to enhance urban drainage. In a public environment such as a parking lot, this rain garden can effectively turn the collected rainwater into a treasure. Potsdamer Platz in Berlin, which accounts for 19% of the total land area, is planned to be water surface, and it can accommodate about 15,000 m3 of rainwater on the surrounding roof and ground. Before the rainwater is injected into the water body of the square, it is necessary to go through a purified pond composed of aquatic plants and cultured microorganisms to complete the purification and secondary filtration of the water to ensure the clean and healthy quality of the water body. The water surface of the square combines with the canal next to the south as an integrated water network ecosystem, creating an urban ecological landscape environment with the theme of water for people living in the downtown.