The terrain gradient plays a critical role in the formation and evolution of landscape patterns. It profoundly influences the spatial distribution and flow of natural and social resources, thereby having a far-reaching impact on ecosystem services and their interrelationships. Taking the Chishui River Basin as an example, six key ecosystem services—water yield, soil conservation, carbon storage, habitat quality, landscape aesthetics, and food production—are evaluated in this study. The study employs an object-based terrain gradient analysis method and spatial overlay technique to investigate the spatio-temporal evolution patterns of ecosystem service trade-offs/synergies. Finally, the XGBoost-SHAP method is introduced to analyze the influencing factors, revealing the contribution and direction of each feature variable. The results indicated that: (1) Between 2000 and 2020, the average values of food production, soil conservation, and water yield increased, while carbon storage, habitat quality, and landscape aesthetics declined, revealing a marked pattern of trade-offs among ecosystem services. (2) Trade-off and synergy patterns exhibited a gradient response: over the past two decades, strong trade-off areas reduced, whereas weak trade-off areas expanded. High synergy areas remained stable or slightly increased in low- to mid-gradient zones (Ⅰ—Ⅲ), but decreased in high-gradient zones (Ⅳ—Ⅴ). Low synergy areas increased in plains and low-hill zones (Ⅰ) and decreased in other terrain classes. The cultivated land reduction and forest restoration promoted weak trade-off and enhanced synergy, leading to a relief in ecological pressure. (3) Vegetation, human activities, and topographic factors were identified as key drivers of trade-off and synergy evolution. NDVI, as a core ecological indicator, played a dominant regulatory role. Intensive human activities exacerbated trade-offs by degrading vegetation health, while topographic factors determined the underlying spatial patterns of ecosystem service distribution. Overall, these components constituted a driving mechanism characterized by natural matrix, ecological outcomes, and anthropogenic disturbance.