Abstract: At the late stage of oil and gas field development, slug flow tends to form in ocean riser systems, and leads to great damage on marine platforms. The slug flow can by eliminated by bellows, which can regulate the flow patterns effectively. In this paper, the numerical model of bellows was built up and its accuracy was verified by performing numerical simulation on internal flow of bellows. Then, the regulating effects of bellows on stratified flow were analyzed and the flow regulating results of bellows with different structural parameters were compared. And finally the structural parameters of bellows were optimized. It is shown that bellows can eliminate severe slug flow by converting the stratified flow into the non-stratified flow. It is an effective passive elimination method. With the certain installation space, the more there are normal bends in bellows, the more frequently the centripetal force direction changes and the more significantly the bellows regulate flow patterns. With the increasing flow rate of the fluid, inertial force is increased, the disturbance caused by the centripetal force is weakened, and the flow at the exits of bellows is distributed less evenly. In the practical engineering, the reasonable value of the bending central angle is 90°, which is convenient for the processing of normal bends and satisfactory for the regulation of flow patterns. The number of normal bends shall be confirmed comprehensively according to the installation space and the investment requirements.