Abstract: An automatic water retaining device comprises two or more automatic water retaining units disposed in parallel, the automatic water retaining unit comprises a panel and a base, wherein the base is provided with a water storage cavity, the panel is rotatably disposed on the base, the water storage cavity is opened and closed through the rotation of the panel, a surface of the panel for covering the base is provided with a floating plate, one end of the base different from the end of the base connected with the panel is provided with a first water inlet communicated with the water storage cavity, and when water enters the water storage cavity from the first water inlet and contacts the floating plate, buoyancy is generated to the floating plate and the floating plate drives the panel to rotate and open under the action of buoyancy.