Polymeric aluminum ferric silicate (PAFSi), which is a composite coagulant, was prepared for high-oil-containing wastewater treatment. X-ray diffraction, scanning electron microscopy, and infrared spectra were used to characterize the structure of the PAFSi coagulants. The characterization results showed that PAFSi was a non-stoichiometric compound of basic iron sulfate salt with three-dimensional cluster structure. COD and oil removal rate were employed as indices for evaluating the coagulation ability of PAFSi in high-oil-containing wastewater treatment. Si/Fe molar ratio of 1:4, Al/Fe molar ratio of 1:1, basicity of 0.5, and preparation temperature of 70 degrees C were favorable to the preparation process. Coagulation tests showed that the optimal conditions for removing high oil concentration were 60-120 mg/L, pH 4-9, and G value of 300-600 s(-1) with 98.2% COD removal and 98.4% oil removal. Zeta potential analysis indicated that charge neutralization was the main mechanism in reducing the surface charge of colloid particles in oily wastewater treatment. In addition, PAFSi presented superior coagulation-flocculation performance in high-oil containing wastewater treatment. This coagulant can be used as an alternative method for pretreating high-oil-containing wastewater in industrial wastewater treatment. (C) 2017 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.