Positive-negative pressure protection device and methane fermentation cylinder

The invention discloses a positive-negative pressure protection device. The bottom end of a first protection tube extends into a cylinder body and below a fermentation liquid surface, the top end of a second protection tube is communicated with the first protection tube, and the bottom end of the second protection tube extends into a water storage pool and below a water surface. The water storage pool is fixed in the cylinder body, and is relatively fixed in the position above the fermentation liquid surface. An atmosphere connection tube is communicated with the first protection tube and a second protection tube respectively, extends from the cylinder body and is communicated with the outside world. When a negative pressure value in the cylinder body exceeds a preset value, air outside the cylinder body enters into the cylinder body through the atmosphere connection tube to avoid internal pressure from decreasing continuously and maintain a pressure value within a certain range. When a positive pressure valu