NOVELTY - Water-gas transposition fermentation for biogas production comprises filling treated water in biogas tank when there is no gas, where bottom of water inlet tank and hydraulic tank is higher than the biogas tank, and pressure of biogas tank is increased when biogas is generated. Part of treated water is pushed by biogas to hydraulic chamber, and a varying gas chamber is formed. Along with gradual increase of generated biogas, the volume of biogas in the biogas tank is gradually increased.    USE - Method of water-gas transposition fermentation for biogas production (claimed).    ADVANTAGE - The method can carry out water-gas transposition fermentation to generate biogas with higher pressure and damages the incrustation formed in the biogas tank automatically. The structure of biogas tank is simple.    DETAILED DESCRIPTION - Water-gas transposition fermentation for biogas production comprises filling treated water in biogas tank when there is no gas, where bottom of water inlet tank and hydraulic tank is higher than the biogas tank, and pressure of biogas tank is increased when biogas is generated. Part of treated water is pushed by biogas to hydraulic chamber, and a varying gas chamber is formed. Along with gradual increase of generated biogas, the volume of biogas in the biogas tank is gradually increased. The volume of movable gas chamber is equal to the increased volume of hydraulic chamber, so that the volume of water and volume of gas is interchanged. An INDEPENDENT CLAIM is included for a method of constructing biogas tank comprising providing biogas tank, which comprises a material inlet tank, a material outlet tank, and a fermenting tank. The bottom of material inlet tank is higher than the top of fermentation tank. The bottom of material outlet tank is lower than the bottom of material inlet tank. The bottom of material inlet tank and the top of fermentation tank are located in the same level. The fermentation tank has a hemispherical structure. The outlet of fermentation tank is higher than the inlet. Multiple single bodies can be connected to increase the volume of fermentation tank.