The invention discloses a diaphragm for a methane tank and a top pressure type methane tank thereof and belongs to the technical field of methane tanks. The diaphragm for the methane tank is of a platy structure, wherein at least one liquid passing pipe is arranged on the diaphragm and is protruded towards the same side of the diaphragm. The top pressure type methane tank comprises a main tank, a concrete foundation and a protecting cover, wherein the main tank is fixedly arranged on the concrete foundation; the protecting cover is fixedly arranged on the upper part of the main tank in a sealing way; a gas overflow pipe is arranged on the protecting cover; the diaphragm is fixedly arranged on the inner wall of the main tank in a sealing way; the projecting liquid passing pipe is toward the concrete foundation; the liquid surface in the main tank is higher than the plane of a lower opening of the liquid passing pipe; and a gas guiding pipe extending out of the main tank is arranged on the diaphragm. According to the top pressure type methane tank, the surface area of the planar diaphragm is a half of that of a spherical collecting bag, and more material cost of a component, labor cost and mechanical energy consumption operation cost are saved.