The invention relates to an urban roof water storage device, belonging to the technical field of domestic drinking water quality advanced treatment. The urban roof water storage device comprises at least one pair of negative and positive electrodes, an electrolysis power source for supplying power to the negative and positive electrodes, and a water container, wherein a water-permeable diaphragm is arranged between the negative electrode and positive electrode in pair; the top of the water container is provided with a window; the negative electrode is arranged on the bottom in the water container; the positive electrode is arranged at the window; the water container is divided by a partition into an upper chamber which communicates with a water inlet pipe and a lower chamber which communicates with a water outlet pipe; and the partition comprises two oppositely arranged porous splints and a physical adsorption filter layer arranged between the two porous splints. When in use, the negative electrode contacts water in the water container, and the positive electrode only contacts the water container through the permeated water of the water-permeable diaphragm. The urban roof water storage device simultaneously adopts the physical and chemical processes to purify water stored in an elevated water tank installed on the top layer of the high-rise building in the urban water supply pipe network.