The karst region in southwest China is known for its vulnerable ecological environment. Investigating the historical patterns of soil erosion and the ecological evolution within this area can offer valuable scientific insights for effective ecological conservation. This study takes the depression deposits in the Zhongliang mountain karst trough valley of Chongqing as the research object, establishes a depth-time model of the sediment profiles using nuclear tracer dating technology, and reconstructs the soil erosion history and ecological change within the small watershed over the past 100 years. The results show that: (1) From 1904 to 1945, the watershed exhibited an erosion modulus of 84 t km^-2 a^-1, characterized by dominant pine forest vegetation and a small population in Beibei, suggesting relatively stable ecological conditions with minimal human-induced transformations. (2) From 1945 to 1958, gradual increases in soil erosion intensity and decreases in vegetation coverage, accompanied by shifts in plant composition towards companion species, likely correlated with population growth in the Beibei area. (3) From 1958 to 1973, the erosion modulus peaked at 344.1 t km^-2 a^-1, with significant increases in companion pollen and sharp declines in Pinus pollen, indicating substantial ecological changes attributed to human activities. (4) From 1973 to 2018, soil loss in the watershed gradually decreased, and vegetation coverage increased significantly, indicating that the ecological environment of the karst small watershed had been significantly improved after years of ecological reconstruction. This study indicates that human activities have been the primary factor impacting the ecological environment of the Zhongliang mountain karst trough valley in the past century. This article also provides some reference for soil erosion research in similar karst areas where long-term monitoring data are not available.