[Objective] This study aims to investigate the spatiotemporal variations and trade-off/synergistic relationships of ecosystem services along altitudinal gradients in the Yarlung Tsangpo River basin，coordinate regional economic development and ecological conservation， and promote regional sustainable development. [Methods] The InVEST model was applied to evaluate the spatiotemporal variations of five critical ecosystem services（habitat quality，water yield，soil retention，carbon sequestration，and food supply）in the basin from 2000 to 2020，with quantitative analysis across different altitudinal gradients. Spearman correlation analysis and Geographically weighted regression were also used to reveal the changes in the trade-off/synergistic relationships among ecosystem services. [Results] 1）From 2000 to 2020，high-value areas of ecosystem service supply are mainly distributed in the lower reaches of the Yarlung Zangbo River basin. Habitat quality maintained consistently high levels，averaging approximately 0.8，accompanied by a significant increase in carbon sequestration. Notably， soil retention capacity demonstrated a substantial increase of 12.43%，while water yield experienced a marginal decline of 1.26%，due to reduced precipitation. 2）Habitat quality，soil retention，and carbon storage exhibited significant increases（p<0.05），with substantial changes observed across 30.24%，42.58%，and 65.40% of the watershed area，respectively. All ecosystem service values declined with increasing elevation，demonstrating distinct altitudinal thresholds：>4 800 m for habitat quality，>3 800 m for water yield，>3 800 m for soil retention，and >2 800 m for carbon storage. 3）Quantitative analysis revealed that food supply showed a weakly increasing synergistic trend with water yield，while soil retention showed a similar trend with habitat quality. The most pronounced synergy was observed between water yield and soil retention（approximately 0.83），while other synergies averaged 0.56. Notably，a significant trade-off relationship was identified between food supply and habitat quality. Spatial analysis demonstrated that synergistic areas between carbon storage and habitat quality constituted the largest proportion of the study area（31.12%，p<0.05）. [Conclusion] The results elucidate the ecosystem service supply capacity across the Yarlung Tsangpo River basin，its altitudinal variation patterns，and the quantitative and spatial trade-offs/synergies among ecosystem services. This research provides a scientific foundation for ecological conservation and restoration.