Abstract: The wastewater COD concentration (3 000-5 500 mg/L) from a refinery refining high naphthenic crude oil is far greater than the design index (2 400 mg/L) of the wastewater treatment plant (WWTP), resulting in incompliance of the discharge standard. By using gas chromatography/mass spectrometry (GC/MS), the effluents of each process unit were analyzed, showing that organic matters in wastewater, such as naphthenic acid, indene ketone, cycloalkene (cycloalkanes) and nitrogen heterocyclic compounds, were difficult to be effectively removed, leading to exceeding of COD discharge standard. Referring to large number of literatures and combining the field pilot test, the processes of biological aerated filter (BAF), hydrolysis acidification and ozone catalytic oxidation were adopted for upgrading of the WWTP. The upgraded processes include oil separation + two-stage flotation + BAF + hydrolytic acidification + A ²O biochemical treatment + membrane bioreactor (MBR) + ozone catalytic oxidation + activated carbon. After upgrading, the operational data showed that the BAF could enhance the anti-shock loading performance of the WWTP, greatly decrease the organic pollutant load, with effluent COD<2 000 mg/L, and make A ²O biochemical treatment and subsequent processing units run stably under the original design conditions. The hydrolytic acidification could enhance biochemical B/C ratio of BAF wastewater, and serve as "water quality stabilizer". The COD removal rate of ozone oxidation tower loading with catalyst was as high as 69.4%, and the effluent COD met the requirements of discharge standards (<60 mg/L).