NOVELTY - Cow dung collecting material liquid is passed to an adjusting tank at 28-37 degrees C. The liquid is acid-precipitated for 7 days under heated environment, to obtain liquid-water mixture. Finally, the liquid-water mixture is passed to a fermenter and fermented at 32-37 degrees C for 25-30 days, to start producing biogas. At this point, biogas fermentation is completed. The produced biogas is passed to a biogas purification equipment through a gas-liquid separator and a water condenser to remove methane in water. Biogas purification is completed.    USE - Fermentation and methane-generation of cow dung liquid by using solar energy (claimed).    ADVANTAGE - The method solves problems that cow dung resources in a cold region are abundant, but thoroughly lose activity caused by reduction of microbial activity, resulting in difficult normal operation of a fermentation system in the cold region. The fermenting and methane-generating technique is high in utilization rate. Both solar energy and sewage gas belong to clean energy sources. The fermenting and methane-generating technique is low-carbon and is applied to heat supply of cold region.    DETAILED DESCRIPTION - Cow dung collecting material liquid is passed through a grille into an adjusting tank. Simultaneously, through a central heat-energy distributing system, the temperature of adjusting tank is adjusted to 28-37 degrees C. The liquid is acid-precipitated for 7 days under heated environment, to obtain liquid-water mixture at 25-40 degrees C with a concentration of 8%. Finally, the liquid-water mixture is passed to a fermenter through a pipe at 35 degrees C. The mixture is fermented at 32-37 degrees C for 25-30 days, to start producing biogas. At this point, biogas fermentation is completed. The produced biogas is passed through a pipe into a biogas purification equipment through a gas-liquid separator and a water condenser to remove methane in water. Biogas purification is completed by removing impurities in gas desulfurization unit. The purified gas is passed through a dispensing device for collecting accumulated methane collected through a pipe for residential use and marsh gas furnace. Biogas is passed through a pipe into a pressure chamber at 20-30 degrees C. The biogas residue slurry is passed to a storage tank and separated in a liquid slag separator. After separation, treated slurry is returned and dry residue is treated.