Abstract: By treating real domestic sewage which had low ρ(C)/ρ(N) in A/O process, the nitrogen removal effect was investigated under different parameter conditions without external carbon source, and the better operating parameter was determined. The average removal rates of ammonia nitrogen (NH₄⁺-N) , total nitrogen (TN) and COD were 100%, 49.48% and 82.03%, respectively, in this situation.The occurrence and development of viscous sludge bulking in this process were also studied. On this basis, the nitrogen removal effect and further development of viscous sludge bulking were investigated under high ρ(C)/ρ(N) condition by putting external carbon sources into the real domestic sewage, then the system reached the best operating parameter. Finally, average ρ(NH₄⁺-N), ρ(TN) and ρ(COD) in effluent met the national level A emission standard, and SVI was also relatively stable at about200 mL/g.