Taking the slope of the Guiwu Expressway as an example, based on on-site investigation and numerical simulation analysis, the mechanism of instability of the slope was analyzed, and improved treatment measures were proposed. The research results indicate that the slope is located on an ancient landslide, and the slope body is extremely soft rock. The excavation of the road cut slope forms a free face, with the front edge yielding and gradually developing towards the back edge, inducing instability. The initial antislip structure is not deeply embedded in the rock, and some antislip piles overturned. So the slope is set as a two-level slope and reinforced with a combination of double row piles, crown beams, and prestressed anchor cables. The safety factor of the slope is numerically calculated to be 1.277, and after treatment, the slope tended to stabilize. Research can provide certain references for similar infrastructure construction.