NOVELTY - Preparing functional bacteria comprises e.g. collecting and screening garbage, sludge, leather factory waste or waste treatment of anaerobic digestion biogas residue into sealed dark anaerobic protein culture medium to obtain functional bacteria contained bacterial liquid, amplifying the obtained functional bacteria contained bacterial liquid into sealed dark anaerobic protein culture medium, and repeating the process to obtain functional bacteria contained bacterial liquid, and inoculating the functional bacteria contained bacterial liquid using anaerobic digestion reactor.    USE - The functional bacteria is useful for improving anaerobic digestion performance of sludge and synchronously enlarging cultivation (claimed).    ADVANTAGE - The method provides functional bacteria in a cost-effective manner.    DETAILED DESCRIPTION - Preparing functional bacteria comprises: (a) collecting and screening garbage, sludge, leather factory waste or waste treatment of anaerobic digestion biogas residue into sealed dark anaerobic protein culture medium to obtain functional bacteria contained bacterial liquid with a protease activity of more less than 100 U/ml; (b) amplifying the obtained functional bacteria contained bacterial liquid into sealed dark anaerobic protein culture medium, and repeating the process for 3 times to obtain functional bacteria contained bacterial liquid with a viable the function of more than 105 colony forming unit/ml and protease activity of more than 300 U/ml; and (c) inoculating the functional bacteria contained bacterial liquid using anaerobic digestion reactor containing sludge for 10-15 days, degrading the functional bacteria in the sludge with protein hydrolysate into low molecular weight organic matters, hydrolyzing the low molecular weight organic matters to form methane by methanogenic bacteria, performing cell proliferation on the functional bacteria strain to obtain sludge digestion residue containing the methanogenic bacteria and the functional bacteria.