Compared with earthen sites in the arid regions of Northwestern China, those located in the humid environments of Southern China are more vulnerable to moisture-related deterioration, exhibiting distinct differences in disease types, characteristics, and development mechanisms. Using the Archaeological Ruins of Liangzhu City as a case study, this research conducted a year-long investigation across nine selected site locations to document site deterioration. More than ten types of deterioration were identified at Liangzhu, among which cracking, surface weathering, mold growth, and faunal damage were the most common. To explore the mechanisms underlying these deteriorations, correlation analysis using SPSS revealed significant correlations between undercut dimensions and environmental factors, with rain erosion as the primary cause driver. Quantitative analysis using R2V software and CAD further demonstrated that rapid reduction in soil moisture content and excavation-induced stress release, are key factors accelerating crack propagation. The Chemical Index of Alteration (CIA) was introduced to assess the chemical weathering of the site soil, revealing a relatively high degree of chemical weathering in the Carp Mountain soil. Ultimately, eight core coupling pathways governing deterioration development were systematically summarized. This research provides valuable insights for the conservation of heritage sites in humid environments.