Abstract: Abstract: Taking into account the characteristics of aquaculture, researchers have proposed the biofloc technology (BFT), and created the conditions in the BFT reactor for the treatment of pollutants. BFT reactor could treat the aquaculture wastewater and the effluents of recirculating aquaculture system (RAS). Total ammonium nitrogen (TAN) and nitrate nitrogen (NO3--N) could be synchronously removed by BFT, and bioflocs could be used as raw materials for aquatic feed. If the BFT reactor was used as the key unit of RAS water treatment, it had the potential to synchronously remove suspended solids (SS), TAN and NO3--N, save water, improve the utilization efficiency of nutrients, and reduce the fixed investment of RAS. A kind of biofloc reactor was designed and applied as the only water treatment device in a pilot-scale RAS, and the operation efficiency was investigated under different hydraulic retention time (HRT), which was 12, 6, 4.5 and 3 h. It indicated that the HRT of biofloc reactor could be as small as 4.5 h. An irreversible phenomenon of irreversible washing out was observed during the 3 h HRT and the experiment could not be continuously conducted. The settling property of bioflocs in reaction zone was generally satisfactory. The sludge volume index (SVI-30) of biofloc significantly decreased with the declined HRT (12, 6 and 4.5 h), which was always higher than 150 mg/L throughout the experiment. The filamentous sludge bulking occurred, the dominant filamentous bacteria (TM7 genera incertae sedis) turned into Haliscomenobacter and Meganema with the declined HRT, and the relative abundance decreased. The waste removal rates were the best when the HRT was 12 h. The lowest concentrations of nitrite nitrogen (NO2--N) and NO3--N were observed under 4.5 h HRT, which were (0.02±0.01) and (1.70±0.06) mg/L, respectively, and for TAN, total nitrogen (TN) and SS in biofloc reactor effluent, the lowest concentrations were observed under 12 h HRT, which were (0.48±0.05), (4.47±1.00) and (14.20±8.26) mg/L, respectively. Besides, it did not cause organic pollution. When HRT was 4.5 h, the reactor could make the concentration of TAN, NO2--N, NO3--N and SS on aquaculture zone lower than 0.76, 0.10, 2.95 and 60.00 mg/L, respectively. Under different HRT conditions, the reactor was dominated by heterotrophic bacteria. The treatment of TAN was mainly through assimilation, and aerobic denitrification bacteria and anaerobic denitrification bacteria were the dominant bacteria in the reactor at the same time. The reactor can obtain a relatively long stable running state and good water treatment efficiency, which can be used as a kind of key water treatment device in RAS. This result can provide a reference for the further research and application of biofloc reactor in RAS.