This paper presents a numerical analysis on the air intake of HIMICO (High-Mach Integrated Control experiment). Mass capture ratio (MCR) of the intake in the supersonic wind tunnel test is different from that we analytically expected quantitatively and qualitatively. CFD results cleared that the reason of the difference is the flow separation on the second ramp surface located at the upstream of the throat. The flow separation is caused by the spillage from clearances between the side walls and 2nd/3rd ramps. Some treatments on the intake configuration such as removing the clearances, adding the additional bleed holes with demerging bleed plenum chamber can improve the inlet performance and prevent the flow separations.