Abstract: This study investigates the aerodynamic performance of a rectangular cylinder with an aspect ratio of 4:1 using the large eddy simulation (LES) method. Within the ranges of angle of attack (α = 0° ~ 12°) and Reynolds number (Re = 100 ~ 120 000). The effects of both parameters on surface pressure, aerodynamic forces, and Strouhal number are systematically examined, together with the evolution of flow patterns and the position of flow transition. The results show that with the increase of the angle of attack, the mean drag coefficient gradually increases, while the fluctuating lift coefficient first increases and then decreases, and the Strouhal number exhibits a complex and irregular variation. When 100 ≤ Re ≤ 1000, the influence of the Reynolds number on aerodynamic force coefficients is pronounced; when 1000 < Re ≤ 120000, the aerodynamic force coefficient is relatively stable and less sensitive to Re. As angle of attack and Reynolds number increase, the upper-side flow successively exhibits three distinct regimes “trailing-edge separation” “separation–reattachment” and “leading-dege separation”, while the lower-side flow exhibits two regimes, namely “trailing-edge separation” and “separation–reattachment.” The regime transitions are closely associated with abrupt variations in pressure coefficient, aerodynamic force coefficients and Strouhal number. As the Reynolds number increases, the flow around the rectangular cylinder progresses through laminar, wake-transition, and shear-layer-transition regimes, while the angle of attack influences the critical Reynolds number of flow transition. When Re = 100, the flow remains laminar for all angles of attack; when Re = 250, increasing the angle of attack causes the flow to evolve sequentially from laminar vortex shedding to wake transition and then to shear layer transition; when Re = 500, wake transition occurs at α = 0°, whereas shear-layer transition dominates for α ≥ 3°; when Re ≥ 1000, shear-layer transition prevail at all angles of attack.