Abstract: Petrochemical wastewater was treated by a hydrolytic/acidification-anoxic-oxic process. The effect of the dissolved oxygen (DO) in oxic reactor on the performance of the process was investigated by detecting the organics, ammonia nitrogen (NH₄⁺-N) and total nitrogen (TN) removal, soluble microbial products (SMP) production and settling property of activated sludge. The hydraulic retention time (HRT) of the process lasted 40hours, and the sludge recycle ratio was 100% during the process. The results showed that the removal of TOC, NH₄⁺-N and TN remained stable when average ρ(DO) in the oxic reactor decreased from 6-7mg/L to 1-2mg/L. SMP was an important component of the residual COD_Cr of the effluent. The contribution of SMP on the residual COD_Cr of the effluent decreased with the decrease of aerobic ρ(DO) from 6-7mg/L to 1-2mg/L in the oxic reactor. Meanwhile, the ratio of protein to polysaccharide in the SMP decreased from 4.6∶1to 0.8∶1. The settling properties of activated sludge became worse with the decrease of ρ(DO), and the sludge volume index (SVI) increased from 62.7mL/g to 136.9mL/g. Fortunately, the water quality of the effluent was not deteriorated with this limited filamentous sludge bulking and there was no sludge loss under this condition. Generally, considering the performance and energy consumption, the recommended aerobic ρ(DO) for the hydrolytic/acidification-anoxic-oxic process was about 2-3mg/L.