NOVELTY - Preparing biomass energy source by anaerobic fermentation, comprises taking restaurant food residues, filtering to obtain filter solution, and using as a liquid culture medium, taking biomass waste, adding water into slurry to obtain semi-liquid culture medium, preparing straw powder into semi-liquid culture medium, placing in a sealed fermenter, regulating the pH, adding fermented mixed cultures or mixed cultures of the biogas slurry additives into the liquid culture medium to form airtight anaerobic fermentation system, filling the fermenter, fermenting, collecting biogas and storing.    USE - The method is useful for preparing biomass energy source by anaerobic fermentation.    ADVANTAGE - The method is simple; and utilizes a simple device, which ensures convenient operation.    DETAILED DESCRIPTION - Preparing biomass energy source by anaerobic fermentation, comprises (i) preparing liquid culture base or semi-liquid culture medium by (ia) taking restaurant food residues, filtering through a filter to obtain filter solution, and directly using as a liquid culture medium; (ib) taking biomass waste comprising food residues, fish bone dregs, vegetable juice, melon peel and other fruit shells solid, adding water into slurry by a disintegrator to obtain semi-liquid culture medium; and (ic) preparing the straw powder into semi-liquid culture medium, where the semi-liquid culture medium comprises (in wt.%) straw powder (10), urea (0.4-0.5), magnesium sulfate (0.1), magnesium sulfate (0.1) and water (remained amount); (ii) placing the culture medium in a vacuumized anaerobic sealed fermenter, using sodium hydroxide (30%) to regulate the pH of the culture medium to 7-7.5; (iii) adding fermented mixed cultures or mixed cultures of the digester biogas slurry additives into the liquid culture medium in the fermenter to form airtight anaerobic fermentation system; and (iv) filling the fermenter with liquid culture medium, fermenting in the incubator or culture room at constant-temperature of 35 degrees C for 1-5 days, collecting the generated biogas, and storing gas in a gas collecting bag for analyzing components.