Taking the southwestern region of China as a representative case, the spatiotemporal evolution and driving mechanisms of terrestrial ecosystem carbon storage were systematically investigated through the integrated application of the InVEST model, Theil index, spatial autocorrelation, and geographical detector. The findings revealed that: ①Terrestrial carbon storage was reduced by 2.048Pg C during 1980~2020, showing a phased characteristic of accelerated decline followed by gradual recovery. The most pronounced reduction was observed between 1990 and 2000, with a distinct spatial pattern of higher values in the west and lower values in the east. Land use conversions were found to have significantly impacted carbon storage before 2000, while this effect was substantially weakened thereafter. The transformation from cropland to forestland was identified as the principal pathway for carbon sequestration, whereas grassland degradation to unused land was recognized as the major route for carbon loss. In addition, cropland conversion to construction land was shown to constitute a persistent pressure on carbon storage. ②Carbon density evolution exhibited a transition from decline to increase, with expanding areas surpassing declining zones as the dominant pattern after 2000. Theil index analysis indicated a general convergence in overall regional carbon density disparities, while heterogeneity at municipal and county levels within provinces was progressively intensified. Spatial aggregation patterns underwent significant reorganization: the western Sichuan Plateau was characterized by a shift from high-value to low-value clustering accompanied by notable northward migration; Tibet transitioned from high-high to low-low aggregation; and a new high-value cluster emerged in the Yunnan-Guizhou border area during 2010~2020. ③ GDP was identified as the dominant determinant of carbon storage variation. The interactive effects among GDP, soil type, and population density with other environmental factors were demonstrated to constitute the primary combination governing regional differentiations in carbon storage. Over the 40-year period, the dominant driving mechanism was observed to transition from spatial dependency-oriented to ecological stress-dominated.