The COD removal of the discharged wastewater of the oi-l field wastewater by bio-flocculant is studied further.The experimental results show that ( 1) COD_Cr concentration is reduced to 134 mg??L from 297 mg??L and the operation costs ??0.7??m³ wastewater under the conditions of pH 5.5 and bio-flocculant in concentration 200 mg??L;( 2) The performance of the bio-flocculant in treating the discharged wastewater of oi-l field is better than that of polyaluminium chloride or polyacrylamide.