Abstract: With the enlarging in the scale of field group and increasing in water cut in produced fluids from old oil fields in Beibuwan area, it is necessary to treat more and more produced water. In this paper, reinjection of produced water is discussed from the point of reservoirs in order to provide basis for reinjection of produced water into the old offshore oilfield. Derivated from material balance equation, the linear relationship between accumulative shortfall (accumulative reinjection) and formation pressure in the reinjected target reservoir has been established. The results show that reinjection of produced water into Weizhou Formation in southern block of WZ-A Oilfield can meet the requirement for reinjection of produced water in Beibuwan Fields.