Abstract: Circular cylinders are widely used in civil engineering. Wind induced large-amplitude vibration often occurs in slender structures with circular cross-sections, on which the aerodynamic forces are significantly influenced by the Reynolds number. The aerodynamic forces, wind pressure distribution, characteristic points of flow around a cylinder, and vortex shedding frequency were studied using rigid model pressure wind tunnel tests to reveal the characteristics of aerodynamic forces on circular cylinders and their mechanism when the Reynolds number ranges from 35k to 330k. Compared with the phenomena seen in sub-critical Reynolds number ranges, within critical Reynolds number ranges, the zero mean wind pressure coefficient points, minimal mean wind pressure coefficient points, and separation points move to leeward, flow around the cylinder becomes asymmetric, the drag coefficient decreases, non-zero lift coefficients occur, and regular vortex shedding disappears.