Anaerobic fermentation device and method for preparing marsh gas

The invention provides an anaerobic fermentation device for preparing marsh gas. The anaerobic fermentation device comprises a tank body, a marsh dreg reflowing pipe and a feeding round pipe, wherein a separation plate is arranged in the tank body; the separation plate is convex downward to form a spherical crown structure and the tank body is divided into a tank body upper part and a tank body lower part; a feeding pipe and a dreg discharging pipe are arranged at the tank body lower part respectively; an overflowing discharging pipe is arranged at the tank body upper part; the marsh dreg reflowing pipe is arranged at the bottom of the separation plate; the tank body upper part is communicated with the tank body lower part through the marsh dreg reflowing pipe; the feeding round pipe penetrates through the separation plate along the vertical direction and is arranged in the tank body; the top of the feeding round pipe is connected with the top of the tank body; the bottom end of the feeding round pipe is conn