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This paper investigates the effects of free-stream turbulence such as longitudinal turbulence intensity (Iu) and integral length scale (Lxu) on the flow fields around, and aerodynamic pressures and forces on a surface-mounted three-dimensional (3D) square cylinder with a height-to-width ratio of 5. The flow fields, aerodynamic pressures and forces were experimentally studied by the particle image velocimetry (PIV) visualizations and pressure measurements in grid-generated turbulent flows and smooth flow. The PIV flow visualizations show that increasing (Lxu) has limited effect on the characteristics of wake flow, except slightly weakens and intensifies the downwash and upwash flows, respectively. The influences of turbulence intensity are noteworthy. The Kármán vortices influence the flow features more significantly in turbulent flows with high intensity. The mean pressures on the side faces of the cylinder decrease as(Iu) increases, but increases on the leeward face. Increasing(Iu) and (Lxu) both enhance the fluctuating lift forces and reduce the mean drag forces at all cylinder spans. The spanwise vortex shedding of the cylinder is respectively enhanced and weakened with increasing ( Iu)and (Lxu)The strong vortex shedding occurs near the cylinder’s mid-span. The analyzed results by Proper orthogonal decomposition (POD) method show the symmetric vortex shedding process on the side faces of the 3D cylinder in smooth flow, resulting in small lift fluctuations. In turbulent flows, the vortex shedding process is symmetric near the free-end, alternating at cylinder’s lower half.