NOVELTY - Preparing biological flocculating agent comprises (1) carrying out dry anaerobic fermentation on the rice straws, (2) filling anti-oxygen and pure nitrogen into the reactor to remove the oxygen content, (3) collecting normal marsh gas tank containing acidogenic bacteria to enrich fermentation and then inoculating in the reactor, (4) centrifuging the fermentation liquor in the reactor, sterilizing the obtained liquor at high temperature and high pressure to obtain a traditional flocculant interleukin (IL) culture medium and (5) inoculating the flocculating bacteria (8%) in the medium.    USE - The method is useful for preparing biological flocculating agent to treat rice straws (claimed).    ADVANTAGE - The method effectively utilizes the rice straw fermentation liquid for 100 days to prepare the biological flocculating agent in a cost effective manner with high yield rate and less energy consumption.    DETAILED DESCRIPTION - Preparing biological flocculating agent comprises (1) carrying out dry anaerobic fermentation on the rice straws (20%) having size of 3-5 cm in a reactor, (2) filling anti-oxygen and pure nitrogen into the reactor for 15 minutes to remove the oxygen content, where the reactor has oxidation-reduction potential rate of below -250 mV, (3) collecting normal marsh gas tank containing acidogenic bacteria to enrich fermentation to 30% and then inoculating in the reactor at 30 degrees C for 80-120 days, (4) centrifuging the fermentation liquor in the reactor for 10 minutes, sterilizing the obtained liquor at high temperature and high pressure to obtain a traditional flocculant interleukin (IL) culture medium with volume ratio of 1:1, where the culture medium has pH value of 0-7.2, and comprises (in g): glucose (10); yeast extract (5); urea (0.5); dipotassium phosphate (5); monopotassium phosphate (2); magnesium sulfate (0.2); and sodium chloride (0.1), and (5) inoculating the flocculating bacteria (8%) in the culture medium, where the flocculating bacteria is obtained by mixing rhizobium radiobacter and Bacillus sphaericus at the volume ratio of 1:1 to form a composite flocculating bacteria.