Biofloc reactor was used as a key unit of RAS (Recirculating Aquaculture System). The effects of different C/N on the wastewater treatment of biofloc reactor were investigated, and four different gradients of C/N ratio(DOC, Dissolved Organic Carbon; DIN, Dissolved Inorganic Nitrogen, C/N=0, 5, 10 and 15) were set. The experimental results showed that:when C/N=0 (without adding carbon source), the efficiencies of nitrogen and phosphorus removal in the reactor were not obvious, and the accumulation of NO^3--N and PO₄^3- appeared. The increase of C/N, the nitrogen and phosphorus removal efficiencies of the reactor were increasing with the rising C/N. The removal efficiency was the best when C/N=15(P < 0.05), and the removal rate (RR) of TN, NO₃^--N and PO₄^3- (RR) were 46.6%, 43.49% and 24.4%, respectively. During the whole experiment, when C/N=0 and C/N=5, the removal efficiencies of SS, TAN and NO₂^--N were significantly higher than that of C/N=10 and C/N=15(P < 0.05). The stability of the reactor gradually became poor with the increasing C/N, and the highest C/N of reactor could be 15. When C/N was 5, the volume index of floc reactor (SVI-30) was sostenuto less than 150 mg/L, and floc expansion did not appear. When 10 ≤ C/N ≤ 15, the stability of the reactor became worse, but it had no effect on the efficiency of nitrogen and phosphorus removal in the reactor. The ORP and DO values were negatively related to the C/N of the reactor, and they can be used as the parameters of the BFT reactor to optimize the process. Overall, the BFT reactor had good water treatment efficiencies on SS, TAN and NO₂^--N under low C/N conditions, when C/N was more than 10, it had a good water treatment performance on TN, NO₃^--N and PO₄^3-, with simultaneous nitrification and denitrification (SND) and phosphorus removal. For BFT reactor used as a key device of RAS, this study can provide reference for further research and application.