A large number of typical dual-structured slopes with loess layer on the upper part and sandstone layer on the lower part are encountered in cutting excavation projects in northwest China with complex geological background. To determine the instability deformation mechanism of such high cutting slope, we built a finite element model for the entire excavation process of the dual structure using GEO-studio with segment K35+092 along the expressway from Lanzhou to Yongjing as research background. The damage characteristics and failure mechanism of loess-sandstone dual structure slope were investigated systematically in terms of the horizontal and vertical displacement and the change of safety factor during staged excavation unloading, as well as the stress and strain of slope in excavation state. The results indicate that the instability and deformation mechanism of loess-sandstone dual-structured high cutting slope is determined by structural characteristics formation lithology. Under the action of excavation disturbance, the displacement of slope crest increased, the plastic strain in soil accumulated, and the steady state of the slope gradually weakened. The weak interlayer in the high cutting slope acts as sliding cushion which plays the role of lubrication and weakening the boundary, thus is a potential destabilizing factor of the slope. With the unloading of the slope, the upper part of the slope, namely, loess, slips and damages the lower slope body; in turn the joint action caused by the shear failure of the lower part caused the slumping of the upper part, resulting in an overall instability of the slope. In order to avoid instability and collapse of similar high cutting slope with dual structures, we put forward some corresponding reinforcement recommendations.