Reaming while drilling is a key step of the casing while drilling process in ocean drilling， while the expandable underreamer can effectively realize casing drilling through control of opening and closing of the reaming arms by pressure difference with large reaming size. The cantilever reamer with locking function in place was designed with the offset jet port arrangement for cleaning the arms. The computational fluid dynamics technology and the standard k- turbulence model were used to simulate and analyze the flow field of the jet ports at different angles and diameters. The simulation results showed that the fluid velocity and the dynamic pressure acting on the cutting teeth of the reaming arms were comprehensively affected by the jet port angle and diameter， and annulus drilling fluid. With the jet port angle α increasing from 40° to 90°， the fluid velocity acting on the cutting teeth changes in a cosine like curve. With the jet port diameter d increasing from 4mm to 10mm， the fluid velocity and the dynamic pressure acting on the cutting teeth change in a parabola shape. Considering the cleaning effect， machining properties and the interaction between the jet fluid and the annulus drilling fluid， optimization was carried out to determine α=70°and d=8mm. The test over land and at shallow sea showed that the structure and the function of the expandable underreamer were feasible， with the reaming size meeting the requirements of normal drilling with casing， and the optimized jet port angle and diameter achieving good cleaning of the arms to effectively prevent bit balling. The research results can provide a reference for the design of underreamers with different specifications in ocean drilling.