NOVELTY - The system comprises a raw material tank, a mixing tank (2), an anaerobic fermenting tank (3), a sedimentation tank (4) which are orderly connected with each other through pipes. The raw material tank is provided with a water feeding/discharging device (5). The mixing tank is provided with a stirring device, which is connected with the water feeding/discharging device. The raw material tank is connected with a carbon-enriched biological raw material tank (11). The carbon-enriched biological raw material tank is connected with rich nitrogen containing biological raw material tank.    USE - The system is useful for producing biogas (claimed).    ADVANTAGE - The system has simple structure and is inexpensive, and greatly improves the fermentation efficiency and yield, reduces the discharging of water and adequately meets the user requirements of rural families.    DETAILED DESCRIPTION - The system comprises a raw material tank, a mixing tank (2), an anaerobic fermenting tank (3), a sedimentation tank (4) which are orderly connected with each other through pipes. The raw material tank is provided with a water feeding/discharging device (5). The mixing tank is provided with a stirring device, which is connected with the water feeding/discharging device. The raw material tank is connected with a carbon-enriched biological raw material tank (11). The carbon-enriched biological raw material tank is connected with rich nitrogen containing biological raw material tank. The water feeding/discharging device is connected to the carbon-enriched biological raw material tank and the rich nitrogen-containing biological raw material tank. The anaerobic fermentation tank is provided with a circulating breaking device. An upper part of the circulating breaking device is provided with a loudspeaker. A lower part of the loudspeaker is connected with a negative pressure pipe. An outlet section of the negative pressure pipe is provided with an electric blade. An electric blade rotating shaft is parallely connected to an outlet section shaft of the negative pressure tube. The negative pressure pipe is provided with an L-shaped pipe whose upper end is connected with the loudspeaker. A straight arm of the L-shaped pipe and a wall of the anaerobic fermentation tank are parallely connected to each other. The negative pressure pipe is provided with a positive injection nozzle. The positive injection nozzle is installed on the straight arm of the L-shaped pipe. The carbon rich raw material tank and the anaerobic fermenting tank are placed on the ground. The nitrogen raw material tank, the mixing tank and the sedimentation tank are placed underground. The water feeding/discharging device of the carbon rich raw material tank is provided with a spiral pusher. The water feeding/discharging device of the nitrogen rich raw material tank is provided with a submersible sewage pump. An INDEPENDENT CLAIM is included for a method for producing biogas.    DESCRIPTION OF DRAWING(S) - The figure shows a schematic view of a system for producing biogas through fermenting mixed raw materials.    Mixing tank (2)    Anaerobic fermenting tank (3)    Sedimentation tank (4)    Water feeding/discharging device (5)    Carbon-enriched biological raw material tank. (11)