The invention discloses a biogas digester which comprises a reaction tank and an arc cover. A gas discharge pipe is arranged at the top of the arc cover; a stirring motor is fixedly mounted on the upper end face of the arc cover close to the gas discharge pipe; the lower end of a motor shaft is fixedly connected with a stirring shaft coaxially; stirring blades sleeve the stirring shaft; a stirring cutter is fixedly mounted at the lower end of the stirring shaft; a feeding pipe is arranged on the side wall of the reaction tank, which is close to an upper end opening; a discharge pipe is arranged on the side wall of the reaction tank, which is close to the bottom; the stirring cutter comprises a pair of upward-overturning blades bent upwards and a pair of downward-overturning blades overturned downwards; the upward-overturning blades and the downward-overturning blades are arranged at intervals; an oblique supporting plate is arranged at the bottom of the reaction tank; the lowermost end of the oblique supporting plate is arranged below the discharge pipe. The biogas digester is provided with the stirring blades and the stirring cutter, and the stirring blades are provided with slanting blades so as to implement rollover on materials; the stirring cutter can smash the materials; combination of the stirring blades and the stirring cutter enables the materials to be more uniformly mixed, so that fermentation efficiency is improved.