Abstract: Taking the BS17 core from the southwestern Sumatra waters as the research subject, this study investigates the grain size composition and its variation patterns of sediments, and reconstructs the evolution history of regional hydrodynamic conditions, sediment supply, and depositional environment since 9 ka BP based on a high-resolution chronological framework. The results indicate that the sediments in the study area primarily originate from three source regions: Sumatra Island, the Andaman Sea, and the Sunda Shelf. The three-end-member components (EM1, EM2, EM3) of grain size respectively represent distal fine-grained components, suspended components from the Sumatra waters, and proximal coarse-grained components. Among these, EM2 has the highest proportion at 55.51%, while EM1 and EM3 account for 23.23% and 21.26%, respectively. A two-stage division can be identified with 3.8 ka BP as the boundary: during the mid-Holocene (9–3.8 ka BP), the sea level was relatively low, the Indian Monsoon and Asian Monsoon were strong, and ocean currents were active, leading to ample supply of distal fine-grained materials. During the late Holocene (3.8–0 ka BP), the sea level rose to modern levels, and the weakened Indian and Asian Monsoons reduced the transport capacity of ocean currents, resulting in a significant increase in the contribution of proximal coarse-grained materials. The response patterns of the low-latitude marine depositional environment to sea level and monsoon changes revealed in this study provide theoretical support for deepening the research on paleoenvironmental evolution in the tropical northeastern Indian Ocean and filling the gap in the mechanisms of global change responses in low latitude marine areas.