Abstract: Large eddy simulation (LES) is used to study the influence of free-end steady slot suction on the aerodynamic forces and flow around a three dimensional finite-length square cylinder with a square cross-section. The aerodynamic forces and the flow around the cylinder have been compared systematically for the suction ratios Q=0, 1 and 3. Special attention is paid on the flow around the free end of the cylinder. It is found that free-end steady slot suction can effectively suppress the aerodynamic forces of the model. The Controlling result is the best when Q=1. Relative to the none control case, the mean drag, fluctuating drag, and fluctuating lift are decreased by 3.92%, 19.08%, and 40.88%, respectively. The mean negative pressure and fluctuating pressure on the free end are most significant at Q=1, which enhances the upwash flow along cylinder side faces and the momentum exchange between free-end shear flow and near wake flow. Thus, this behavior effectively suppresses the spanwise vortex shedding and the aerodynamic forces of the cylinder; at the same time, the flow separation and reattachment occurred on the top of the cylinder. When Q=3, the flow separation in most areas of the top surface of the cylinder is completely inhibited.