NOVELTY - Method for preparing metal sulfide nanoparticles, involves (a) adding sulfate reducer to S2-generation culture liquid to obtain solution (A), (b) adding precursor metal salt of easily hydrolyzable metal ion to organic solvent to obtain solution (B), (c) mixing the obtained solution (A) and (B), culturing the solution and generating deposition at the bottom of the bottle, and (d) centrifuging and collecting the deposit, removing the dissolved metal ion and culture medium, and drying the mixture in vacuum to obtain metal sulfide nanoparticles.    USE - The method is useful for preparing metal sulfide nanoparticles (claimed) and green nanoparticle.    ADVANTAGE - The method is simple, safe and environmentally-friendly, requires less energy, and does not require biological reduction-chemical depositing coupling method.    DETAILED DESCRIPTION - Method for preparing metal sulfide nanoparticles, involves (a) adding sulfate reducer to S2-generation culture liquid to obtain solution (A), (b) adding precursor metal salt of easily hydrolyzable metal ion into organic solvent and dissolving the mixture to obtain solution (B), (c) mixing the obtained solution (A) and solution (B) in one container, hermetically culturing the solution at 25-45 degrees C until concentration of the metal ion is not reduced continuously, stopping the reaction and generating deposition at the bottom of the bottle, and (d) centrifuging and collecting the deposit generated at the bottom of the bottle after reacting, removing the dissolved metal ion and culture medium, and drying the mixture in vacuum to obtain metal sulfide nanoparticles. The content of sulfate reducer is 5-20 vol.%. The concentration of metal ion in the organic solution in step (b) is 0.01-0.1 mol/l. The precursor metal salt of easily hydrolytic metal ion is soluble in organic solvent and insoluble in water. The volume ratio of solution (A) and solution (B) is 1:0.25-1:4. The solution with lower density is added to the solution with higher density, when the solution (A) and solution (B) are mixed. An INDEPENDENT CLAIM is included for enrichment culture method of liquid of sulfate reducer, which involves transferring anaerobic sludge collected from digesting pool of sewage treating factory into container with enrichment culture liquid of sulfate reducer, adding 5 g of anaerobic sludge in every 500 ml of enrichment culture liquid of sulfate reducer, culturing the container at 35 degrees C, shaking the container more than once every day, obtaining 150 ml of upper sulfate reducer liquid in the fixed days to access to the novel enrichment culture liquid of the sulfate reducer, implementing the enrichment culture and obtaining 50 ml of upper sulfate reducer liquid in the fixed days to access to the novel 500 ml enrichment culture liquid of the sulfate reducer to obtain sulfate reducer liquid, where the sulfate reducer liquid is applied for normal storage and seed bacterial suspension. The enrichment culture liquid of the sulfate reducer comprises lactic acid, acetic acid or malic acid (0.1 mol), sodium sulfate (0.1 mol), ammonium chloride (1), potassium dihydrogen phosphate (0.5), magnesium sulfate (0.5), calcium chloride (0.1), yeast powder (0.5) and deionized water (1000 ml). The pH of the culture medium is 7.