In early October 2021, Shanxi was hit by heavy rainfall, and the rammed earthen walls on the west side of the Pingyao Ancient City, a world cultural heritage site, were damaged to varying degrees. To clarify the disease characteristics and development mechanism of rammed soil on the inner wall after being affected by precipitation, typical disease characteristic segments were selected for classification, statistics, and quantitative characterization. The types of rammed soil diseases mainly include cracks, sheet-like erosion, excavation, biological damage, and collapse. In addition, the engineering characteristics of compacted soil were obtained through indoor macro and micro tests, and statistical regression analysis was performed on the quantitative characterization results to obtain the mechanism of disease occurrence and development. It was found that 1) the formation of cracks was related to the plastic deformation caused by freeze-thaw cycles; 2) the occurrence of sheet-like erosion was negatively correlated with the content of clay particles; 3) the occurrence of excavation in the middle and lower parts of the wall was positively correlated with the content of soluble salts; 4) for collapse, one part was caused by the development and evolution of various diseases, and the other part was caused by the decrease in cohesion between compacted soil particles after rainwater infiltration. Finally, a disease framework system for the rammed soil on the west side of the Pingyao Ancient City was established, providing a scientific basis for its scientific conservation and targeted restoration and reinforcement.