Across-wind loads and their responses are key to the design of super-tall buildings. Clarifying the action mechanism of lift force on a square cylinder helps understand the characteristics of wind loading and raise new aerodynamic control methods. Three-dimensional large eddy simulation (LES) is adopted to investigate flow patterns around and aerodynamics of a square cylinder at a uniform inflow, full incidences .and a Reynolds number of 22000. The relationship between flow structures, pressure fields, and aerodynamic forces is discussed. Special attentions have been paid on the dynamic characteristics of separation bubbles and their influence mechanism on the lift of the cylinder. The present numerical results are consistent with the wind-tunnel experiments. There are unsteady separation bubbles at one side of the cylinder at incidences 10°~20°. Multiple vortices are observed behind the separated shear layer, and would collapse, reattach, or re-separate then. The single side separation bubble causes a reduction of drag and an increase of Strouhal number, resulting in local strong negative pressures and a high instantaneous cross-wind lift.