Fluidized-carriers were supplemented into the aerobic tank of a full-scale wastewater treatment plant (WWTP) using an anaerobic/anoxic/aerobic (A2/O) system to improve the nitrogen removal efficiency in effluents. The effects of carrier supplementation on denitrification ability and the bacterial community structures were investigated over 10 months. The results showed that the average effluent concentration of total nitrogen (TN) was maintained at 9.46 ± 1.14 mg/L, which was lower than 15.17 ± 2.00 mg/L in the effluent without carrier supplementation, indicating that adding fluidized-carriers into the aerobic tank contributed to nitrogen removal efficiency. A thick biofilm was formed after 4 months, which provided a good anoxic-aerobic microenvironment to the microbes. Illumina sequencing analysis showed a higher bacterial diversity in the biofilm. The relative abundance of nitrifying bacteria, denitrifying bacteria, and aerobic denitrifying bacteria in the biofilms was 13.68–39%, 11.56–12.17%, and 9.76–12.50%, respectively, which was beneficial for nitrogen removal in the system. The most prevalent genera were Nitrospira, Bacillus, Thauera, Hyphomicrobium, Acinetobacter, Zoogloea, Pseudomonas, and Paracoccus, which can metabolize nitrogenous or aromatic compounds and were the major functional bacterial genera, suggesting that these organisms play key roles in biodegradation processes in the carrier-added A2/O wastewater treatment system. Display omitted •Carriers in aerobic tank of full-scale A2/O system contributed to nitrogen removal.•The effluent TN was decreased significantly after carrier addition for 4 months.•Fluidized-carriers were beneficial for the accumulation of functional microorganisms.•Nitrospira and Bacillus genera developed in biofilm than that in suspended biomass.