The invention relates to a water oil storage device with a mooring function and an assembly method thereof. The oil storage device is characterized by comprising an oil storage tank, a mooring system, an output system and an oil transportation system; the oil storage tank comprises a buoyancy cabin, an oil storage cabin and a ballast cabin which are arranged from top to bottom; a top oil inlet is arranged in the side wall in the upper part of the oil storage cabin, an upper oil baffle is arranged below the top oil inlet, an oil inlet is arranged in the middle part of the upper oil baffle, oil film is connected with the lower part of the oil inlet in a sealing way, and a lower oil baffle is arranged on the lower end of the oil film; a plurality of oil separation interior rings and oil separation external rings are arranged inside and outside the oil film from top to bottom at intervals; a plurality of guide rails are vertically arranged on the bottom of the upper oil baffle, and each guide rail passes through the through hole in each oil separation external ring; a plurality of top water inlets are arranged in the side wall of the oil storage cabin below the upper oil baffle; a plurality of bottom water inlets are arranged in the bottom of the oil storage cabin; the mooring system comprises a mooring buoy connected with the ballast cabin; a plurality of mooring cables are connected with the lower part of the mooring buoy; one end of an oil transportation pipe of the output system passes through the buoyancy cabin and enters into the oil storage cabin, and the other end of the oil transportation pipe is connected with a boat stopping buoy; and one end of an oil transportation pipe of the oil transportation system is connected with a seabed oil extraction system, and the other end of the oil transportation pipe is embedded into the oil storage cabin through the top oil inlet.