The centralized treatment method is a widely used form of wastewater treatment that tends to be less effective at removing toxic substances. Therefore, a detailed analysis of the composition of wastewater can provide important information for the design of an effective wastewater treatment process. The objective of this paper was to investigate particle size distribution (PSD), biodegradability, and the chemical composition of the petrochemical wastewater discharges. For this purpose, this project selected the petrochemical wastewater and treated wastewater of China National Offshore Oil Corporation Zhongjic Petrochemical Co. Ltd. as the analysis objects. The step-by-step filtration method, along with a molecular weight classification method, was selected to build the chemical oxygen demand (COD) and biochemical oxygen demand (BOD) fingerprints of petrochemical wastewater and treated wastewater.The results showed that the main pollutants were settleable particles in petrochemical wastewater, which contributed to over 54.85% of the total COD. The colloidal particles with particle sizes in the range of 450-1000 nm had the highest COD value in the treated wastewater, which contributed 34.17% of the total COD of treated wastewater. The results of the BOD analysis showed that the soluble fractions were the main reason that treated wastewaters did not meet the treatment standards. Tests on the organic compounds in petrochemical wastewater found that there were mainly linear paraffins, branched paraffins, benzene series compounds, and some plasticizers in the influent of the petrochemical wastewater. The most abundant pollutants in treated petrochemical wastewater were the adjacent diisobutyl phthalate and the linear alkanes. Fourier transform infrared (MR) transmission spectroscopy analysis showed that the settleable particles of petrochemical wastewater and membrane bioreactor (MBR)-treated wastewater contained multiple types of organic substances. The results also indicated that removing the oil-settleable substances, the colloidal particles (450-1000 nm), and the soluble organics will be necessary for the treatment of petrochemical wastewater. (C) 2018 The Chemical Industry and Engineering Society of China, and Chemical Industry Press. All rights reserved.