A new biological filter reactor with multi-layer fiber ball was designed to overcome the disadvantages of traditional aquaculture, such as the high energy consumption and the complicated process. This reactor removed SS and nitrogen pollutants without changing water, and achieved a high concentration of dissolved oxygen in the effluent water. The results show that the naturally-formed biofilm built an externally-aerobic and internally-anoxic environments in the fiber. Therefore, the reactor could simultaneously accomplish the nitrification process and denitrification process. The influence of different water recirculating rate was discussed. In the case of water circulation rate of three times a day that was the optimal for the reactor, the concentration of NH⁺₄-N in effluent could be reduced to 0. 2 mg/L with the removal rate of 96. 15%. The removal efficiency ofNO^-₃-N was stable at about 70%, and the concentration ofNO^-₂-N in effluent could be reduced to 0. 1 mg/L with the removal rate of 95. 82%. Besides, the concentration of COD in effluent decreased to 4. 0 mg/L and the removal rate reached 60% or higher. At the same time, the SS removal rate was up to 100%. The simultaneous nitrification-denitrification process in the reactor was successfully achieved, and this study provides a new idea for the recirculating aquaculture system.