Abstract: The flow around a finite-length square prism is numerically investigated using the LES (large eddy simulation). The aspect ratio of the prism is 5. The prism is mounted on a flat wall, with one end free. The thickness of the boundary layer on the flat wall is very small to be negligible. The Reynolds number based on the free-stream oncoming velocity and the prism width is 3 900. Based on the simulation results, it is found that the near wake is highly three dimensional under the effects of the free-end downwash flow. Two typical flow states were observed in the near wake: first, the spanwise vortices are staggered arranged similar to the Karman vortex street; second, the spanwise vortices are quasi-symmetrically arranged without obvious periodicity. These two typical states occur alternately, which affects the aerodynamic forces on the prism.