The utility model discloses a multilayer mechanical stirring device for a large anaerobic fermentation tank. The device comprises an anaerobic tank and a stirrer, the stirrer comprises a central stirrer and a near-wall stirrer; the central stirrer is positioned in the center of the anaerobic tank; the central stirrer comprises a central rotating shaft connected with the top and the bottom of the anaerobic tank, and composite blades arranged on the central rotating shaft; the near-wall stirrer is close to the side of the anaerobic tank; the near-wall stirrer comprises a near-wall rotating shaftand axial flow type blades arranged on the near-wall rotating shaft; the composite blades can drive liquid in the anaerobic tank to move up and down when rotating under the stirring working condition, the axial flow type blades can drive liquid in the tank to move up and down when rotating under the stirring working condition, and the center stirrer and the near-wall stirrer are matched to enablethe liquid in the anaerobic tank to form circulation. By driving the liquid in the anaerobic jar to flow circularly, the anaerobic jar has a strong convection effect and a strong turbulence diffusioncapability, and the liquid stirring operation effect is good.