NOVELTY - Producing feed additive from livestock breeding sewage and purifying livestock breeding sewage into recycled water, comprises (i) carrying out anaerobic/aerobic treatment of livestock breeding sewage, (ii) purifying the methane solution into recycled water by a membrane bioreactor and an photobioreactor, and (iii) pressurizing the concentrated algae extract, separating algae extract by the filter, adding activated bacterial strain and complex enzyme for fermentation/enzymolysis, and producing microalgae feed additive, adding in a dosing machine and mixing, and then preparing animal feed.    USE - The method is useful for producing feed additive from livestock breeding sewage and purifying livestock breeding sewage into recycled water.    ADVANTAGE - The method has high degree of economic adaptability and zero emission.    DETAILED DESCRIPTION - Producing feed additive from livestock breeding sewage and purifying livestock breeding sewage into recycled water, comprises (i) washing the excrement water produced in the live stock farms and urine excreted by animals, mixing in a sewage manifold of the livestock breeding field, discharging mechanical impurities e.g. large floating objects by the grid and entering into an aerobic biochemical reactor, adjusting the water quantity, water quality and pH of the sewage, adding the partial activated sludge produced by the front part of the subsequent anaerobic biochemical reactor to the rear part of the acidulated adjusting pool, decomposing long chains and macromolecule organic matters in the sewage into short chains and micromolecule organic matters, and improving the biochemical property of the livestock breeding sewage, separating excrement residue from the sewage as compost by a solid liquid separator, conveying the acidulated sewage into an anaerobic biochemical reactor for anaerobic biochemical treatment, and collecting the sewage gas produced in the anaerobic biochemical reactor in a gas storage kettle, adding the methane solution processed by the anaerobic biochemical reactor to an aerobic biochemical reactor for aerobic biochemical treatment, reducing the biochemical oxygen demand (BOD) and chemical oxygen demand (COD) of the methane solution, and converting the ammonia nitrogen in the methane solution into nitrate or nitrite easily absorbed by micro algae, and also eliminating the peculiar smell of methane solution, (ii) placing the aerobic biochemical processed methane solution in a settling tank for precipitating, collecting the bottom sludge in a sludge storage tank, filtering and pressing the sludge by a plate frame filter press as compost, and returning the filtrate into the anaerobic biochemical reactor for continuously processing, introducing the supernatant in the settling tank in the membrane bioreactor for further aerobic treatment, filtering by an ultrafiltration membrane in the membrane bioreactor and discharging suspensions and bacteria, discharging regularly the sludge produced by the membrane bioreactor in the settling tank or making up to the aerobic biochemical reactor as activated sludge, collecting the ultrafiltration solution produced by the ultrafiltration membrane in the membrane bioreactor in the sterilizing tank, adding disinfectant, then adding neutralizer and neutralizing the disinfectant and then entering into the photobioreactor, culturing microalgae as nitrogen and phosphor source, adding algae and small amount of nutrient salts to the photobioreactor, after ventilating sewage gas and combusting to produce carbon dioxide waste gas, performing a closed photoautotrophic high density culturing of microalgae, then using the microalgae to absorb heavy metal elements e.g. nitrogen, phosphor, zinc and copper in the nutrient solution, after passing one microalgae growth period, discharging algae extract from the photobioreactor and entering a flocculation and sedimentation tank, adding flocculant to the tank, precipitating, and collecting the supernatant as recycled water, (iii) pressurizing the concentrated algae extract flowing out of the bottom of the flocculation precipitation tank and then entering into the filter, collecting the filtrate in the tank as recycled water, separating algae extract by the filter directly entering into a fermentation/enzymolysis tank, adding the activated bacterial strain and a complex enzyme for fermentation/enzymolysis, and then producing microalgae feed additive, adding the microalgae feed additive and the animal regular feeds without addition of heavy metal elements e.g. zinc and copper in a dosing machine and mixing, and preparing the animal daily feed.