Abstract: As more and more offshore oilfields step into the middle and late stages of development, reservoir pressure depletes greatly, which results in high production decline rate and impacts oilfields’ development benefits seriously. Waterflood development is the best method to supplement the formation energy and ensure the stable oilfield production. In this paper, a kind of large-bore separate-layer sand control string suitable for offshore water injectors was researched to deal with the severe sand production and high water injection rate of offshore oilfields. Based on the optimal design of key string tools, the full bore of sand control string is increased from Ø120.65 mm to Ø152.4 mm and the central pipe of separate-layer water injection string is increased from Ø88.9 mm to Ø114.3 mm. This string can improve the maximum water injection rate of single well and solve the problems caused by the limited wellhead pressure in offshore oilfields, such as limited water injection rate and unqualified oil reservoir injection allocation. In addition, it is beneficial for the later well workover and abandonment operations, and provides the reference for the design of separate-layer sand control string.