As a cornerstone structural material in Chinese architectural heritage, understanding the long-term durability of Chinese fir (Cunninghamia lanceolata) is critical for its preservation. This study comparatively investigates the degradation of Chinese fir under three accelerated aging protocols simulating key environmental threats: ultraviolet (UV) weathering, thermal-humidity cycling, and salt fog. The results revealed distinct degradation mechanisms with significant implications for heritage diagnostics. Salt fog exposure induced the most severe degradation, causing a 63.9% loss in bending strength through comprehensive chemical attacks. UV weathering led to significant surface photodegradation and microcracking, while thermal-humidity cycling caused a 46.5% reduction in bending strength, primarily due to physical stresses. Crucially, a strong, universal correlation was established between cellulose crystallinity (CrI) and the mechanical properties (tensile, R = 0.874; compressive, R = 0.902; bending, R = 0.941) across all aging conditions. This identifies CrI as a robust and minimally invasive indicator for assessing the mechanical integrity of historic timbers. Furthermore, the degradation kinetics under each stressor followed highly predictable linear trends (R² > 0.89), providing a quantitative basis for developing targeted conservation strategies and more accurate service life prediction models for timber heritage structures.