Abstract: Combined with partial nitrification and biological fluidized bed，The process was started with low C/N synthetic wastewater，and the influence on the nitration and nitrosation process was studied by the concentration of COD，ammonia nitrogen， DO and pH in sewage． The study showed that a shorter HrT and less seed sludge was better for the growing of biofilm，and could successfully realize the fast start of biological fluidized bed． High concentration of ammonia nitrogen in influent could contribute to the accumulation of nitrite in reactor， but this accumulation was not stable． When pH in reactor was around 7. 5 ～8. 1， amd DO was about 1. 5 ～2. 5 mg /L，the maximum rate of nitrosation could reach 75%，and the ammonia nitrogen removal rate was more than 85%． The concentration of NO－ 2 －N and NO－ 3 －N in effluent decreased with the increasing concentration of COD in influent． When the concentration of COD in influent was 50 mg /L，the concentration of nitrate reduced sharply and the concentration of nitrite reduced slowly，nitrogen removal by simultaneous nitrification and denitrification occurred in reactor．