The east gate wall of the outer city of the Shimao site has suffered from serious outward deformation and is facing the risk of collapse. In order to protect it scientifically, the stability and reinforcement methods for the wall were studied. The weathering reduction coefficient was proposed to characterize the degree of weathering of the mortar joint. The discrete element method was used to establish the wall model and verify its effectiveness. The deformation and stability of the wall under weathering were studied by the strength reduction method, and the weathering degree of the wall and the mechanical parameters of the mortar joint were determined by the inversion analysis method. The results show that the main cause of wall deformation and instability was the decrease of mortar joint strength. Under the action of earth pressure, the wall failure presented a slip-overturning mode. At present, the wall is in an understable state with the weathering coefficient of 0.45-0.48, mortar joint strength cohesion of 21 kPa and internal friction angle of 14°, respectively. On this basis, two reinforcement methods—grouting and lateral supporting—were proposed and compared, to finally obtain good results. The research results could provide a reference for the follow-up reinforcement and protection of the city wall of the Shimao site.