Abstract: To investigate the microbial community characteristics of sludge bulking caused by Microthrix parvicella, the dynamics of microbial community, nitrifying bacteria and filamentous bacteria were studied using an anaerobic-anoxic-oxic (A²/O) reactor and high-throughput sequencing technology. Results show that sludge bulking caused by M. parvicella was induced when the operating temperature decreased from 24℃ to 14℃ for 21 days, and the sludge volume index (SVI) value increased from 81 mL/g to 197 mL/g. The average removal rates of COD and NH₄⁺-N were 84% and 98%, respectively. Sludge bulking changed the microbial community structure, which the relative abundance of Actinobacteria increased significantly from 8% to 20% while Proteobacteria and Nitrospirae decreased. The abundance of ammonia oxidation bacterium (AOB) and Nitrospira decreased, while the abundance of Nitrobacter increased at low temperature, however, there was still a certain amount of nitrobacteria in the system, which maintained good nitrification performance. Additionally, the relative abundance of M. parvicella, Thiothrix and Flavobacterium increased, while the other eight filamentous bacteria decreased. Sludge bulking can be controlled by increasing the temperature.