With the continuous development of the Bohai oilfield, some old oilfields have exceeded the design life of the oilfield and entered the abandonment stage. At the same time, due to the management of low efficiency wells and long shutdown wells in the oil field, the number of well abandonment operations for the purpose of reusing well slots is increasing. In the process of scraping, cleaning, abrasive water jet cutting, etc. in oil and gas wells, a large amount of sewage will be generated, and the cost of transporting it back to onshore for disposal is high. Therefore, the treatment of oily sewage has become an urgent problem to be solved. By analyzing and comparing three kinds of waste treatment units such as crawler-type negative pressure vibrating screen and flocculation filter-press integrated machine and oil-based drilling cuttings thermal treatment device commonly used in offshore platforms, this study comprehensively considers marine environmental protection laws and regulations, assesses the measures to utilize the existing platform to dispose of oily waste liquid on-site, and develops a small skid-mounted high-efficiency separation device for oily sewage. The oily sewage stored in the upstream oil storage tank is pumped to high-efficiency separation equipment, and after conditioning, flocculation, air flotation, filtration and other technical treatment measures, the treated sewage oil content reaches 90%, and the treated sewage oil tank is transported by tugboat back to shore. The treated effluent has an oil content of only 0.04 mg/L, which can be used as the well washing recycling fluid. This study provides effective technical support for the disposal of oily sewage from well abandonment on offshore platforms. The high-efficiency separation device and sewage on-site treatment and reuse technology for oily sewage has been applied in Bohai oilfield with good results. On average, saving in treatment cost of a single well is up 460 000 yuan. This study provides a new way for the onsite treatment of sewage for offshore platforms without production processes, such as drilling jackets and abandoned wells.