A combined system of moving bed biofilm beactor (MBBR) - algae reactor is designed as a biological treatment unit to eliminate inorganic nitrogen in aquaculture water and solve the problem of nitrate accumulation in recirculating aquaculture system(RAS) production, taking the RAS of Litopenaeus vannamei as an example (culture load 5000 kg). According to the principle of material balance, it is determined that the influent flow of the biological treatment unit is 453.3 m3 / h and the MBBR size is 4 m × 4 m × 2.8 m (4), algae reactor size is 6 m × 6 m × 2.8 m (4), the hydraulic retention time of MBBR is 0.3 h, the hydraulic retention time of algae reactor is 0.36 h, the renewal amount of fresh water in the system is 0.97 m3 / h, the number of cycles is 22 times / d, and the nitrate nitrogen in the system can be maintained within the safe concentration range below 70 mg / L. The experimental system was constructed for verification. It was found that compared with MBBR, the removal rate of total nitrogen increased from 3.9% to 42.8%; at the same time, algae can absorb phosphate through photosynthesis. The removal rate of total phosphorus by the combined system is as high as 66.8%, and the SGR of algae is 3.86% - 10.35% / day. The combined system can effectively shorten the hydraulic retention time required to remove the same amount of nitrogen and phosphorus. The research can provide a theoretical reference for nitrate in situ elimination technology and the establishment of a biological treatment unit in RAS, and promote the green development of aquaculture.