Abstract: In this paper, the effect of flow around a wall-mounted 2D square cylinder on WSS (Wall Shear Stress) in a TBL (Turbulent Boundary Layer) is studied using TR–PIV and hot wires. The Reynolds number, defined by the cylinder width and the free stream velocity, is fixed at 1.1 × 10⁴. There exist two types of large-scale flow structures, i.e., vortex I which denotes those moving towards the wall, and vortex II which denotes those moving downstream. The flow structures have significant effects on the flow and WSS: increasing the streamwise velocity gradient that results in a sharply increased WSS; reducing the streamwise velocity gradient that results in a sharply decreased WSS; and changing the WSS direction. The study is helpful to understand the physics of surface erosion, pollutant accumulation, flow energy loss, etc.