The invention belongs to the technical field of biogas digester structures, and particularly relates to a biogas digester. A material drawing-out pipe is arranged on the upper part of the other side of a digester body, and a liquid draining pipe is formed at the bottom end of the digester body; an annular limiting groove is formed in the digester body, a material box is mounted in the annular limiting groove, and liquid draining holes are formed in the bottom end face of the material box; the material box is slidably mounted in the digester body through a lifting stirring device; a driving motor is detachably mounted on a movable cover, the output end of the driving motor penetrates through the movable cover to be connected with a telescopic rotating shaft, and the bottom end of the telescopic rotating shaft is connected with a cavity rotating column; the cavity rotating column is movably arranged in a sleeve lead screw, a ball nut sleeves the sleeve lead screw, and the ball nut is mounted on the bottom end face of the material box; the bottom end of the rotating column is connected with a stirring shaft, and a plurality of stirring rods are mounted on the stirring shaft; and strawand liquid excrement are subjected to separated fermentation. The phenomenon that the fermentation time of straw waste and the fermentation time of the excrement are different, and consequently fermented gas is wasted is prevented.