This article utilizes three-dimensional numerical simulations on the flow around a cold cylinder with a constant Reynolds number （Re=80） and a Richardson number （Ri） ranging from -5 to -0.5， to investigate the formation mechanism of the wake structure and the influence of buoyancy on the wake structure. The results indicate that within the range of -2.5≤Ri≤-0.5， the wake structure around the cold cylinder exhibits a consistent cyclic pattern due to the influence of buoyancy. AsRireaches a certain threshold （e.g.， Ri=-5）， the cyclic pattern of the cold cylinder flow undergoes a change. With the decrease of Ri， the overall wake structure of the cold cylinder deflects in the direction of gravity， while the number of rib structures exhibits a non-monotonic variation， and the wall-attached flow of the cylinder wake intensifies. When Ri decreases to a certain threshold， flow separation occurs solely from beneath the cylinder. The lift and drag coefficients， along with the Nusselt number on the cylinder’s surface， increase monotonically as Ri decreases.