Abstract:  When the ocean riser is subjected to current of a certain velocity, flow around cylinder and vortex-induced vibration occur around the riser, causing serious damage to the safety of the riser. Two-dimensional numerical simulation of the flow around cylinder of the riser under subcritical Reynolds number is carried out. The parameters such as the resistance coefficient and the lift coefficient of the cylinder are investigated. Especially, the flow around a riser with an additional pipe is simulated, and the simulation results show that the additional pipe can help to suppress the flow around cylinder. This research has a certain reference value for the development of the offshore riser with additional pipes.