Summary: Numerical study of the flow past two tandem cylinders is carried out at \(P/D = 1.5\) and 2.5 for \(Re =2.8 \times 10^{5}-7.0 \times 10^{5}\). The shear-stress transport \(k-{\omega}\) turbulence model is selected to capture the flow characteristics around the cylinders. This paper focuses on the characteristics of the fluid field, hydrodynamic forces and vortex-shedding frequencies at two cylinder configurations for different Reynolds numbers \((Re)\). Qualitative and quantitative comparisons with the published data are performed to evaluate the current results and reasonable agreement is obtained. The results show that vortex shedding occurs behind both the upstream and downstream cylinders at \(P/D = 2.5\) for the entire region of testing \(Re\), which is significantly different from the most critical gap spacing (above 3\(D)\) at low \(Re\). The drag direction changes from negative at \(P/D = 1.5\) to positive at \(P/D = 2.5\). And the fluctuations found in the lift for the downstream cylinder are more drastic than that for the upstream cylinder, which indicates that the downstream cylinder may behave in large vibration. The Strouhal number \((St)\) at \(P/D = 1.5\) is relatively low compared to that at \(P/D = 2.5\) due to the strong interactions between two cylinders with small gap spacing.