The invention discloses a system and a method for harmless treatment of livestock and poultry breeding wastewater. The system comprises a wastewater treatment system and a sludge treatment system, wherein the wastewater treatment system comprises a water collecting pool, a solid-liquid separator pool, an air flotation precipitation pool, an acidification pool, a UMIC anaerobic reactor, a circulation tower, an air floater, an ammoniation pool, a first precipitation pool, a nitrification pool, a second precipitation pool, an aeration pool, a third precipitation pool and a storage pool, wherein the water collecting pool, the solid-liquid separator pool, the air flotation precipitation pool, the acidification pool, the UMIC anaerobic reactor, the circulation tower, the air floater, the ammoniation pool, the first precipitation pool, the nitrification pool, the second precipitation pool, the aeration pool, the third precipitation pool and the storage pool are sequentially communicated. According to the present invention, with the method, livestock and poultry breeding wastewater is subjected to harmless treatment by using the UMIC dual internal circulation anaerobic reactor as the coretreatment component through the physicochemical and biochemical combined method, such that the concentration of the water pollutants after the treatment can reach the requirements of the Discharge Standard Of Pollutants For livestock And Poultry Breeding (GB18956-2001), the treated water can be directly discharged so as to be used for the irrigation of surrounding crops after the treatment standard is achieved, and a certain economic benefit is provided for the surrounding farmers.