Abstract: The reverse osmosis concentrated water from double-membrane process of petrochemical plant is hard to treat for its high COD, poor biodegradability and high salinity. The test found that good results can be obtained by ozonation – photoelectrocatalysis combined technique. In this paper, three factors, namely O3 concentration, pH and temperature, which influence the treatment efficiency of the concentrated water, were examined. The results show that COD and oil concentration in the effluent are less than 50 mg?L-1 and 0.5 mg?L-1, respectively, when the wastewater with COD of 200-250 mg/L, oil concentration of 15-18 mg/L is treated under the conditions of the optimal pH value of 9, the O3 dosage of 16 mg/L, preaeration 90 min in O3 and reside time of 60min in photoelectrocatalysis unit. The qualities of the treated water meet the requirements of the enterprise.