Abstract: To clarify the mechanism of aerodynamic interference between two closely spaced staggered circular cylinders, a large eddy simulation (LES) method is adopted to investigate the flow structure and aerodynamics of the two cylinders at a high Reynolds number (Re=1.4×10⁵). The ratio of center-to-center pitch (P) to the diameter of the cylinder is P/D=2. The correlations of aerodynamic coefficients between the two cylinders are analyzed, and the relationship between the aerodynamic forces and the flow structure is discussed as well. A new explanation is proposed for the mechanism of the large net lift on the downstream cylinder. It is revealed that the present numerical results are in a good agreement with those by a wind tunnel test. For small incidence angles (0°~10°), the downstream cylinder experiences a mean negative drag force, which is induced by the recirculation flow and the gap flow between the two cylinders. For the incidence angles of around 10°, the downstream cylinder is subject to a large mean lift, which is ascribed to the combined effects of three factors, i.e. the shift of stagnation point, the high-speed gap flow between the two cylinders, and the early separation and reattachment of shear layer of the downstream cylinder.