The effects of boundary layer conditions on the aerodynamic forces of a finite-length square cylinder were experimentally investigated in wind tunnel. The width of the tested model d = 200 mm, and the aspect ration H/d = 5. The tested Reynolds number based on oncoming flow velocity (U∞) and d ranged from 0.68×105~5.47 ×105. It was found that the Reynolds number has no obvious effects on the aerodynamic forces of finite-length square cylinder in both uniform flow and turbulent boundary layer. The local aerodynamic forces coefficients ,   and   are significantly different along cylinder span. Both   and   are remarkably increased in the turbulence boundary layer relative to those in uniform flow. The bi-stable phenomenon of the fluctuation pressure and aerodynamic forces observed in uniform flow disappears completely in turbulent boundary layer. Contributions from the symmetrical and anti-symmetrical pressure distribution modes to the pressure fluctuation are qualified based POD analysis. In uniform flow, pressure fluctuation near both ends of the cylinder is dominated by symmetrical pressure mode, while that at cylinder mid-span is dominated by anti-symmetrical pressure mode. In turbulent boundary layer, anti-symmetrical pressure distribution is prevalent at all spansiwe positions. For both boundary layer conditions, only the anti-symmetrical POD mode bears remarkable periodicity, which determines the periodicity of the aerodynamic forces. No obvious periodicity is observed for the symmetrical POD mode, though it is dominant at some spanwise positions.