The flow around bluff bodies has always been a hot topic in fluid mechanics, but few researches have been carried out on the flow around cylinder with racks, which are the core supporting structures of jack up. k-ω SST urbulence model was adopted to conduct numerical simulation research on the three-dimensional cylinder with racks at Reynolds number 3900, including the characteristics of the time averaged flow field of lift, drag coefficient and pressure coefficient, as well as the instantaneous flow field characteristics such as velocity distribution and vorticity distribution. The simulation results show that the vortex shedding occurs in the flow field around the structure. In the wake area near the structure, the velocity distribution of the flow field presents a "V" shape, and the "V" shape bottom tends to be flat at a little distance, showing a "U" shape, and a velocity backflow area opposite to the flow direction occurs behind the cylinder with racks. It is also found that the flow along the structure at different locations presents obvious three-dimensional characteristics.