Abstract: The unsteadiness in shock wave and boundary layer interactions is one of foundation problems in the aerodynamic design of high-speed vehicles.Most previous researches have focused on laminar and turbulent interaction.The intrinsic physical origin of separation shock low-frequency oscillation is still under debate.There exist two utterly opposite theoretical explanations, upstream influence and downstream influence.The analysis of unsteady motion in shock wave and transitional boundary layer interactions are helpful to aware of the effects of boundary layer state and separation bubble structures on low-frequency oscillation, which providing an insight to point out new direction for forcing mechanism.A numerical analysis of unsteady motion in shock wave and transitional boundary layer interaction for a 24 deg compression ramp at Mach 2.9 is performed by the mean of direct numerical simulation.The blowing and suction disturbances are added upstream at specified stream wise locations to induce the interaction of shock wave with early stage of transitional boundary layer in compression ramp.Firstly, the reliability of the used program is verified.Secondly, the intermittency and oscillation of shock motion are then analyzed in detail.Through analysis of power spectral density of wall pressure signals, effects of separation bubble structure on unsteady motion are studied.Finally, the physical mechanisms of low-frequency oscillation are initially discussed.Results indicate that the unsteady shock motion is highly intermittent, the characteristic of shock oscillation is low-frequency.The time scale is about 10 times the magnitude of fluctuating signals in the incoming boundary layer.Three dimensional structure of separation bubble has little effect on the low-frequency unsteadiness.Based on the low-pass filtered instantaneous flow fields, evidence is found of a correlation between the low-frequency oscillation of shock and the contraction/dilation of separation bubble in the downstream.