The rack cylinder is an important part of the pile leg structure of the jack up platform. Because of its complex structure, the flow field around the rack cylinder is different from that around the ordinary cylinder, which brings difficulties to the research of the rack cylinder. In this paper, using CFD(Computational Fluid Dynamics) solved the flow field of chords with different rack height and rack width under different KC and Re, the characteristics of the flow field around the cylinder with rack are obtained. The results show that Re, KC, rack height and rack width all have different effects on the flow field. When Re and KC are constant, Cd will increase with the increase of rack height ratio, the change of Cd and Cl is not significant, while the change of Cd and Cl varies with Re when the chord structure is fixed.