In order to reduce the computational cost of LES for high Reynolds number, a hybrid Large Eddy Simulation/Reynolds-Averaged Navier-Stokes (hybrid LES/RANS) approach which combined two-equation k-ω SST turbulence model and Yoshizawa's one-equation sub-grid scale model with a blending function was developed. The hybrid method together with AUSM⁺-up scheme was conducted to investigate a Mach 2.85 flow field of a 20-degree compression ramp. The effects of two kinds of inflow boundary conditions (fixed inlet and inlet with super-imposed white noise) were investigated. Separation and reattachment of the boundary layer along with separation shock were obtained from the simulation. The separation zone predicted was significantly larger than the experimental result. Defects of the hybrid method were analyzed and possible improvements were presented.