NOVELTY - Water pressure floating cover type methane tank comprises a cover body comprising a cover, a cover wall and an air guide pipe, a distance interlayer between cover wall and a container wall, a device I, a device II and a locating device. The cover body is covered on inner bottom of a container. The device I is used for introducing liquid material into the cover. The device II is used for controlling the height between the inner plane of cover in an air storage chamber and liquid level to be larger than 0. The locating device is used for freely floating cover body and a slag discharging device.    USE - Used as a water pressure floating cover type methane tank.    ADVANTAGE - The floating cover type methane tank solves the existing technical problems of high manufacturing cost on methane tank, incapable of automatically discharging slag or automatically stabilizing pressure so that the people can obtain renewable energy sources (methane) when processing large amount of waste e.g. straw and household waste at low cost (claimed).    DETAILED DESCRIPTION - Water pressure floating cover type methane tank comprises a cover body comprising a cover, a cover wall and an air guide pipe, a distance interlayer between the cover wall and a container wall, a device I, a device II and a locating device. The cover body is covered on the inner bottom of a container. The device I is used for introducing liquid material into the cover. The device II is used for controlling the height between the inner plane of the cover in an air storage chamber and the liquid level to be larger than 0. The locating device is used for freely floating the cover body and a slag discharging device. The material inlet is higher than the zero-pressure liquid level. The cover wall at least equals to the value obtained through dividing the largest output methane pressure of vertically upwards force area storage chamber in the cover by average area liquid density of the liquid level transverse cover wall. An INDEPENDENT CLAIM is also included for increasing and decreasing largest output methane pressure by the water pressure floating cover type methane tank, comprising adding heavy substance or reducing counter weight of the cover body based on initial counter weight of the cover body.    DESCRIPTION OF DRAWING(S) - The diagram shows a schematic representation of the water pressure floating cover type methane tank.