The process design of a certain FPSO production water system mainly depends on the water centrifuge, but the actual treatment effect is not ideal. With the increase of production water, the existing wastewater treatment system needs to be reformed to meet the environmental assessment. Through the study, the existing two unqualified crude tanks of FPSO are used instead of the slop tank, and the two slop tanks and two fuel tanks are transformed into two stage production water sedimentation tanks. And the oil collection function of the production water settling tank is added to the two rainwater tanks, so as to achieve the purpose of buffering and expansion of the production water treatment processing and provide new method for function adjustment of FPSO in service.