Abstract：It is common for highway and railway to share the same slope in foothill and mountainous areas. The soil slope there suffers double dynamic loadings，one is the vehicles running on the highway on the top of the slope，and the other is the train trucks acting on the bottom of the slope. According to the loading situation，the variations of maximum unbalance force in the soil elements is discussed，and some interactive problems between the dynamics loading and soil slope are analyzed by means of theoretic analysis and numerical simulation. The study result shows that (1) the effect of vehicle loading on the instability of slope is greater than that of train truck loading，(2) in the same loading situation and environment，in the stability analysis process，the effect of train truck loading on the slope can be ignored for the soil slope with small inclination，and (3) for this sort of soil slope，there would be the potential sliding wedges in the middle areas of the slope surface. Finally，phenomenon of displacement lagging is found in analysis，which appears within the moving soil elements，and seems more evidently in the initial phases before limit equilibrium. In term of the theory of constitutive relations and the property of lagging ring under the circle loading conditions of soil，it is very easy to explain the phenomenon. Because of displacement lagging，the sliding tracks of instable soil wedges become more complicated，and further study is needed to solve this problem.
