The utility model discloses a double-tube type overflowing system of a biogas fermentation tank. The system comprises the biogas fermentation tank, an overflowing tube, an overflowing device and a pipeline bracket, wherein one end with an overflowing hole of the overflowing tube tilts downwards and is communicated with the biogas fermentation tank; the other end of the overflowing tube is communicated with the overflowing device; a biogas slurry tube a and a biogas slurry tube b are arranged at the bottom of the overflowing device; a biogas slurry inlet of the biogas slurry tube a is lower than a biogas slurry inlet of the biogas slurry tube b; and a gate valve and an electronic valve are mounted on the biogas slurry tube a. Compared with a conventional overflowing system, the system adopts an operation mode that a material is firstly fed and subsequently discharged, and due to adoption of the device, activated sludge inside the fermentation tank can be effectively retained, and the fermentation efficiency is improved. When the electronic valves have faults and need to be repaired, the biogas slurry can be discharged in time through another standby pipeline, particularly the pipeline is prevented from being blocked, the biogas slurry is discharged according to time sequence control, the activated sludge inside the fermentation tank can be retained, and the fermentation efficiency is improved.