It has been shown in existing engineering practices that for four-lane tunnels, due to the large excavation width of the cross-section and uneven distribution of surrounding rock loads, problems such as excessive vault settlement and uncontrolled sidewall displacement are prone to occur during construction. If there is a pre-excavated cutting slope on the lateral side of the tunnel, the superposition of the slope unloading effect and tunnel excavation disturbance will further intensify the redistribution of surrounding rock stress and increase the risk of instability, which urgently requires targeted theoretical analysis and technical support.Taking a separated highway tunnel project in Zhuhai as the background, the impact of the construction of a four-lane tunnel on the stability of the surrounding rock of the cutting slope at the tunnel top and adjacent lateral road sections was studied through FLAC3D numerical simulation combined with on-site monitoring data. The results show that the installation of retaining walls and reinforced anchor lattice beams on the lateral cutting slope effectively restrains the lateral displacement of the slope, and a certain protective effect is exerted on the surrounding rock of the left tunnel to be constructed subsequently. Subsequently, the excavation of the left tunnel section and the construction of the initial support cause the redistribution of surrounding rock stress, resulting in deformation characteristics dominated by the convergence of the surrounding rock at the tunnel vault and under the inverted arch toward the tunnel. After the initial support forms a closed loop, the vault settlement deformation of the tunnel section is the largest, followed by the inverted arch heave deformation. The maximum lateral displacement of the surrounding rock on the right side is greater than that on the left side, but the maximum lateral displacements of both sides are significantly smaller than the maximum values of settlement and heave deformations. Since the maximum values of these four deformations of the surrounding rock in the tunnel section are all less than the specification limits, the tunnel surrounding rock is in a stable state. After the secondary lining is constructed, continuous distribution of tensile stress still exists in the surrounding rock over a large area of the vault, among which the tensile stress development area inclined upward on the side of the cutting slope may directly reach the ground surface; however, compared with the situation after the initial support forms a closed loop, the secondary lining further optimizes the stress distribution of the surrounding rock, with the maximum values of shear stress and maximum principal stress reduced by 9.1% and 1.6% respectively, and the surrounding rock stress values are less than the strength index, indicating good tunnel stability. In addition, attention should be paid to the fact that the tensile stress area of the vault surrounding rock directly reaching the ground surface may become a channel for rainwater infiltration, and the surrounding rock of the tunnel on the lateral slope has a tendency of outward lateral displacement.