The invention discloses an A<2>/O-BCO improvement process utilizing fermented sludge, and belongs to the technical field of biological treatment of sewage. The process comprises the following steps: mixing fermentation liquid with domestic sewage in an A<2>/O for anaerobic reaction, mixing the fermentation liquid and domestic sewage mixture with nitrification liquid for denitrification and dephosphorization reaction under an anoxic condition, carrying out aerobic treatment, then separating the material from a plate sedimentation tank, and respectively obtaining supernate and sludge; placing the supernate into a biological contact oxidation pond with carbon fiber, carrying out short-range nitrification reaction under an aerobic condition, putting reaction liquid subjected to the nitrification reaction into a sedimentation region, and respectively obtaining nitrification liquid and sludge; mixing the sludge separated from the plate sedimentation tank and the sludge obtained from the sedimentation region with alkaline solution for fermentation, and obtaining fermentation liquid. According to the A<2>/O-BCO improvement process, weight reduction and recycling of the sludge can be realized in a sewage treatment process, and the trouble that low C/N is poorer than the sewage treatment effect due to shortage of carbon sources is solved.