The Nanjing City Wall is the largest existing city wall in the world and it is important to evaluate its safety scientifically. The portion of the wall leaning against mountain is one of the main forms of the Nanjing City Wall, the loading characteristics of which are different from those of the self-bearing city wall because it is connected with soil. Our study summarizes six typical diseases of the portion of the wall leaning against mountain, and proposes a numerical modeling method for it to simplify it as a city wall-bonding interface-soil model. Using ABAQUS software, and in combination with corresponding engineering examples, a pre-warning analysis of overall collapse was carried out on the bulging deformation of the wall. The results show that with the decrease of soil strength, the displacement of the wall increases continuously, forming a bulge that will lead to collapse. [The pre-warning of deformation value can be established through the obvious inflection point on the city wall bottom tensile stress-top displacement curve.] Based on this value, the influence of the soil slope height on the overall collapse is discussed, and a reasonable value of slope angle is given. The soaking and swelling of the city wall bottom could reduce the strength of the bottom and result in a large relative displacement there. A warning can be alarmed when the bottom wall has the ultimate tensile strength based on observed maximum relative displacement. Finally, suggestions for the restoration and conservation of the wall are proposed.