Abstract: When a supersonic or hypersonic flow propagates over a compression ramp, a remarkable adverse pressure gradient may appear near the corner due to the displacement effect of the wall, possibly leading to a separation in the vicinity of the corner, as well as the potential emergence of the shock-boundary-layer interaction. The separation could result in a variety of unfavorable factors in engineering applications. An effective tool to reduce flow separation is to introduce a steady suction slot on the wall upstream of the corner. Based on the triple-deck theory under the large-Reynolds-number asymptotic framework, this paper studied the mechanism of the suppression effect of the wall suction on the laminar-boundary-layer separation. It is found that, as long as the suction slot is arranged at a position O(R^-3/8L) upstream of the corner, the do-minant parameter that determines the suppression effect is the suction flux, which is irrelevant to the location of the suction slot; changing the width and velocity of the suction slot simultaneously but keeping the suction flux unchanged does not affect the suppression effect on the separation zone.