NOVELTY - Preparing biological methane gas, comprises pulverizing raw materials in a digester, scaling component of the raw materials to carbon-nitrogen ratio of 20-30:1, adjusting pH to 6.8-7.5, and stirring them and then liquefying the mixture, and processing liquefied product in marsh gas tank to remove sand, controlling the oxygen concentration in the digester to a preset concentration conditions, introducing the liquid into a anaerobic fermentor for anaerobic fermentation, and adjusting fermentor temperature to below the anaerobic fermentation tank fermentor preset temperature.    USE - The method is useful for preparing biological methane gas (claimed).    ADVANTAGE - The method is capable of simply preparing the biological methane gas with high yield and quality.    DETAILED DESCRIPTION - Preparing biological methane gas, comprises pulverizing raw materials in a digester, scaling component of the raw materials to carbon-nitrogen ratio of 20-30:1, adjusting pH to 6.8-7.5, and stirring them and then liquefying the mixture, and processing liquefied product in marsh gas tank to remove sand, controlling the oxygen concentration in the digester to a preset concentration conditions, introducing the liquid into a anaerobic fermentor for anaerobic fermentation, adjusting fermentor temperature to below the anaerobic fermentation tank preset temperature, heating the anaerobic fermentation tank until the preset temperature is equal to the anaerobic fermentor, when the heating temperature is stopped, adding desulfurization agent in the anaerobic fermentor for simultaneously desulfurization with fermented raw materials, where the desulfurizing agent is hydrogen sulfide and the mass ratio of desulfurizing agent to the fermentation material is 1-600:800, stirring the fermentation raw material at a speed of less than 0.5 m/second, connecting the anaerobic fermentor to back of biogas digester through a pipe, cleaning the anaerobic methane fermentation tank water, introducing gas into a buffer tank, maintaining the pressure in the buffer tank to 0.8-1.2 Mpa, absorbing the gas in the buffer tank to 400-600 L/hour through a bottom of an absorber, maintaining the column pressure to 0.8-1.2 Mpa, flowing the gas from a top of the absorber at 100-200 L/hour, where methane content in the biogas is 85%, dissolving the carbon dioxide in water and then flowing into the bottom of the absorber, introducing the gas from the gas inlet into the dewatering tank through the bottom of the tank, and storing the gas in a storage tank.