The invention discloses a double-pipe overflow system of a biogas fermentation tank. The double-pipe overflow system comprises a biogas fermentation tank, an overflow pipe, an overflow device and a pipeline support, wherein the end, where an overflow port is located, of the overflow pipe is inclined downwards and communicated with the biogas fermentation tank, the other end of the overflow pipe is communicated with the overflow device, the bottom of the overflow device is provided with a biogas fluid pipe a and a biogas fluid pipe b, a biogas fluid inlet of the biogas fluid pipe a is lower than that of the biogas fluid pipe b, and the biogas fluid pipe a is provided with a sluice valve and an electric valve. Compared with an existing overflow system, the double-pipe overflow system has the advantages that by adopting an operating way of discharging and then feeding, active sludge in the tank can be effectively intercepted, and the fermentation efficiency can be improved. When the electric valve has a malfunction or needs to be repaired, the biogas fluid also can be discharged in time through another standby pipeline, the blockage of the pipelines can be prevented, the biogas fluid can be discharged according to the sequential control, the active sludge in the biogas fermentation tank can be intercepted, and the fermentation efficiency can be improved.