Abstract: The effects of different CaO₂ concentrations on the acid production performance of anaerobic fermentation of excess sludge were studied and the microbial mechanisms were analyzed. The experimental results showed that CaO₂ promoted the hydrolysis of sludge. When the dosage was 0.35 g CaO₂/g VS, the fermentation effect was the best. The output of SCFAs reached 1424 mg·L^-1, about 4.3 times that of the control group. The oxidative damage caused by CaO₂ to bacterial communities is proportional to its concentration. The results of high-throughput sequencing showed that CaO₂ would reduce the diversity of microbial communities, enrich acidogenic bacteria and inhibit other bacteria. The relative abundance of Firmicutes increased from 7.16% in the control group to 64.23% in the 0.35 g CaO₂/g VS group. The significant increase in the abundance of Clostridia and Bacilli was the main reason for the high yield of SCFAs in this group. With the increase of CaO₂ concentration, bacterial extracellular polymer (EPS) transforms from tightly bounded EPS (TB-EPS) to loosely bounded EPS (LB-EPS) and slime EPS (S-EPS), promotes the synthesis and secretion of bacterial extracellular polysaccharide, and improves the viability of functional bacteria. The antioxidant enzymes of bacteria also affect the changing of functional bacteria community in anaerobic fermentation system. The addition of CaO₂ improves the relative abundance of SOD and CAT related genes, improves the antioxidant capacity of functional bacteria by promoting the secretion of antioxidant enzymes, and enhances the production of SCFAs.