NOVELTY - Producing methane gas, involves adding cysteine into culture medium purged with nitrogen, heat-preserving, transferring to anaerobic bottle and sterilizing to obtain medium, then inoculating Clostridium butyricum, culturing to obtain C.butyricum fermentation liquid, then naturally drying fresh pig manure, crushing, adding C.butyricum fermentation liquid, dried pig manure and water into anaerobic culture tank, sealing, forming fresh biogas slurry, then adding water to anaerobic culture tank and continuously fermenting to obtain the product.    USE - The method is useful for fermenting and producing methane gas (claimed).    ADVANTAGE - The method improves productivity of methane by 84.21% and finally increases marsh gas production rate by 19.02%.    DETAILED DESCRIPTION - Method of fermenting and producing methane gas, involves preparing Clostridium butyricum fermentation liquid by adding cysteine into culture medium purged with nitrogen, heat-preserving for 15 minutes, then transferring to anaerobic bottle or anaerobic tube and sterilizing at 115 degrees C under high pressure for 15 minutes to obtain medium, then inoculating C.butyricum and culturing at 35-40 degrees C for 12-16 hours to obtain Clostridium fermentation liquid, where the culture medium comprises 5 g/L tryptone, 5 g/L beef extract, 10 g/L yeast extract powder, 5 g/L glucose, 0.5 mL/L resazurin, 40 mL/L salt, 0.25 g/L calcium chloride, 0.5 g/L magnesium sulfate, potassium dihydrogen phosphate and dipotassium hydrogen phosphate, 10 g/l sodium bicarbonate, 2 g/l sodium chloride and 0.5 g/l cysteine, then naturally drying fresh pig manure, crushing into 40 mesh sieve size, adding 131.58 g C.butyricum fermentation liquid, 500 g dried pig manure and 684.21 g water into 5 L anaerobic culture tank, sealing with solid paraffin, maintaining at low temperature of 37.13 degrees C for 27 hours, forming fresh biogas slurry, then adding water to anaerobic culture tank and continuously fermenting to obtain the product.