Abstract: The rock masses comprising the slope at the portal of a twin-arched tunnel in the town of Fuxi, Anhui province are strongly affected by weathering and unloading. Before excavation, the slope had noticeable down-slope deformation at its middle and upper levels and the deofrmation was in the state of limit equilibrium. On the basis of the site investigation and the study of the rock mass structures, the paper esbalishes the deformation mode and failure mechanism of the slope. Furthermore, the paper simulates the deformation character FLAC3D. The deformation analysis leads the design measure that uses grout to stabilize the rock masses. The result of calculations indicates that the slope can have better stability after grouting. In the meanwhile, the monitored data also shows that the stability of the grouted slope is better than that of the slope before grouting. This can ensure that the expressway will be safe during the road construction and its long-term usage.