NOVELTY - The device has down-flow anaerobic fermentation tank (2) and up-flow anaerobic fermentation tank (14). The two ends of liquid upper conveying pipe (5) are respectively connected with up-flow anaerobic fermentation tank and upper side portion of post storage tank (8). The two ends of post storage tank precipitated sludge backflow pipe (7) are respectively connected with circulation backflow pipe (3) and post storage tank. The lower portion of post storage tank is equipped with lower tapered hopper (12) having dreg discharging port valve (13).    USE - Mixed sludge backflow methane generating device.    ADVANTAGE - The simple structure of device produces high methane fastly using fast anaerobic fermentation with complete liquid fermentation and hence the reliable operation of device is simple and convenient.    DETAILED DESCRIPTION - The device has liquid lower conveying pipe (15) that is provided with liquid conveying pump. A valve is arranged at lower side portion of down-flow anaerobic fermentation tank and up-flow anaerobic fermentation tank. The two ends of circulation backflow pipe are respectively in fixed connection with upper side portion of down-flow anaerobic fermentation tank and liquid lower conveying pipe. The discharging port and feeding port of up-flow anaerobic fermentation tank discharging backflow pipe (1) are respectively in communication with up-flow anaerobic fermentation tank and liquid lower conveying pipe. The down-flow anaerobic fermentation tank and up-flow anaerobic fermentation tank are respectively provided with post storage tank at the outer portion. The two ends of liquid upper conveying pipe provided with fermentation liquid discharging port valve (4) and fermentation liquid feeding port valve (6) are respectively connected in communication with up-flow anaerobic fermentation tank and upper side portion of post storage tank. The two ends of post storage tank precipitated sludge backflow pipe are respectively connected with circulation backflow pipe and post storage tank. The upper discharging port valve (9), lower port discharging valve (11), post storage tank precipitated sludge backflow pump (10) and sludge backflow valve (16) are orderly mounted on post storage tank precipitated sludge backflow pipe. The lower portion of post storage tank is equipped with lower tapered hopper having dreg discharging port valve.    DESCRIPTION OF DRAWING(S) - The drawing shows an explanatory view of the mixed sludge backflow methane generating device.    Up-flow anaerobic fermentation tank discharging backflow pipe (1)    Down-flow anaerobic fermentation tank (2)    Circulation backflow pipe (3)    Fermentation liquid discharging port valve (4)    Liquid upper conveying pipe (5)    Fermentation liquid feeding port valve (6)    Post storage tank precipitated sludge backflow pipe (7)    Post storage tank (8)    Discharging port valve (9)    Post storage tank precipitated sludge backflow pump (10)    Lower port discharging valve (11)    Lower tapered hopper (12)    Dreg discharging port valve (13)    Up-flow anaerobic fermentation tank (14)    Liquid lower conveying pipe (15)    Sludge backflow valve (16)