To clarify the mechanism that two side-by-side circular cylinders will experience biased flow patterns, flow and aerodynamics of two side-by-side circular cylinders of various center-to-center spacing ratios (P/D) were investigated using large eddy simulation (LES) at a high Reynolds number of 1.4×10⁵. Efforts were devoted to study the characteristics of the biased flow and its influences on the aerodynamics of the two cylinders. Results revealed that the present numerical results are in good agreement with those by the wind tunnel test. With the spacing increase, three flow patterns were found, i.e., single-bluff-body pattern, biased flow pattern, and parallel vortex street pattern. When P/D=1.1, the two cylinders underwent the single-bluff-body flow pattern and biased flow pattern intermittently. There was a significant difference between flow field characteristics and aerodynamics under these two flow patterns. When P/D=1.2~1.5, the two cylinders underwent biased flow pattern. The flow field of the wake region and the aerodynamic forces were asymmetrical for the two cylinders. The direction of the biased flow switched intermittently from one side to the other. The intensity of the vortex shedding was weak, and the magnitude of the fluctuating forces was low. When P/D=2~4, aerodynamic interference between two cylinders was weak and there were two vortex streets behind the two cylinders, which resulted in large fluctuating forces.