NOVELTY - A fluid fermenting cellar comprises a sealing cap (1) and a cement ground (5). The sealing cap (1) is made of flexible sealing film material and the lower part edge is an end-to-end annular aerating sealing ring (2). The cement ground (5) corresponding to the aerating sealing ring (2) is embedded with an annular recess (3).    USE - The fluid fermenting cellar is used for producing marsh gas (claimed).    ADVANTAGE - The fluid fermenting cellar is used for producing marsh gas at high yield and at low cost. The fluid fermenting cellar has simple device structure, easily manufactured, and inexpensive.    DETAILED DESCRIPTION - A fluid fermenting cellar comprises a sealing cap (1) and a cement ground (5). The sealing cap (1) is made of flexible sealing film material and the lower part edge is an end-to-end annular aerating sealing ring (2). The cement ground (5) corresponding to the aerating sealing ring (2) is embedded with an annular recess (3) whose cross section is inverted O-shape with small opening and large chamber. The aerating sealing ring (2) and the annular recess (3) are combined. The aerating sealing ring (2) is aerated to firmly combine the sealing cap (1) and the ground surface (5) to form a sealed solid space to become the fluid fermenting cellar insulated from the outside. INDEPENDENT CLAIMS are:    (1) a method of producing marsh gas by fluid fermenting cellar by packaging the pre-treated, stacked, prepared and mixed fermenting raw material in the plastic boxes (barrels); trimly stacking and piling the plastic boxes (barrels) containing the fermenting raw material in the fluid fermenting cellar to fulfil the fluid fermenting cellar; covering the sealing cap to the outside of the plastic box (barrel) containing the fermenting raw material, combining the aerating sealing ring at the lower edge of the sealing cap with the annular recess embedded on the cement ground, after aerating the aerating sealing ring, the sealing ring and the ground being firmly connected to form the fluid fermenting cellar to receive the fermenting raw material; after performing the leakage detecting, determining that the fluid fermenting cellar has no air leakage; conveying the marsh gas produced from the fluid fermenting cellar into an accordion air bellow typed fluid air storage sac by the outlet pipe and the air conveying pipe, then pressurizing by the air pump and conveying the marsh gas to the air storage tank by a purifying device; after finishing producing the gas, pumping the remained marsh gas in the fluid fermenting cellar by an air pump, discharging out the gas in the aerating sealing ring, opening a sealing cap to remove the plastic boxes (barrels) having the remained dregs to finish a marsh gas production period; and    (2) a method of producing marsh gas by soft cover marsh gas pool by putting the pre-treated and prepared fermenting raw material in the open marsh gas pool from its upper opening and pressing the raw material; stacking the fermenting raw material in the open marsh gas pool, after becoming mature, adding in water; adjusting the pH value and sealing the pool, combining the aerating sealing ring at the soft cover edge with the annular recess embedded on the open marsh gas pool, after aerating the aerating sealing ring, firmly connecting the soft cover and the open marsh gas pool to form a soft cover marsh gas pool; after the leakage detection is qualified, the fermenting raw material fermenting and producing gas in the soft cover marsh gas pool; using the traditional material liquid backflow or marsh gas backflow method to stir; the marsh gas produced in the soft cover marsh gas pool being conveyed to the accordion air bellow typed fluid air storage sac by the outlet pipe and the air conveying pipe, then pressurizing by the air pump and conveying the marsh gas to the air storage tank by a purifying device.    DESCRIPTION OF DRAWING(S) - The drawing shows a schematic view of a fluid fermenting cellar.    Sealing cap (1)    Sealing ring (2)    Annular recess (3)    Cement ground (5)