Located on the southwestern margin of the South China Sea (SCS), Borneo provides a key geological window into the tectonic evolution of Paleo-Pacific Subduction and SCS.The Sintang-Semitau region, situated at the convergence of NW Borneo, SW Borneo, and Sibu-Miri tectonic zone, preserves a complete Paleozoic to Cenozoic multi-stage tectonic-magmatic-sedimentary record. This region is a key area for deciphering the tectonic evolution of West Borneo. However, previous studies predominantly focused on the Mesozoic magmatism and sedimentation, lacking the study on the Cenozoic sedimentary evolution and associated source-to-sink systems. To address this issue, we conducted systematic stratigraphic, petrographic, and in-situ detrital zircon U-Pb-Hf isotopic analyses on well-exposed clastic sandstones from the Eocene Melwai group (Payak Formation), Suwang group (Dangkan sandstone and Silat sandstone) and Teu Formation in the Sintang-Semitau region. The detrital zircons from the Payak Formation primarily exhibit age peaks at ~105 Ma and ~124 Ma, with corresponding εHf (t) values of +4.6 to +7.3 and +5.8 to +7.5, respectively. In the Dangkan sandstone, detrital zircon populations show dominant age peaks at ~86 Ma, ~115 Ma, and ~130 Ma, and yield εHf (t) values of +2.7 to +10.7, +6.3 to +9.5, and +4.2 to +15.6. The Silat sandstone displays major zircon age peaks at ~77 Ma and ~116 Ma. For the Teu Formation, characteristic age peaks occur at ~87 Ma, ~117 Ma, and ~263 Ma, with εHf (t) values of +4.3 to +11.3, -4.6 to +7.5, and -13.0 to -8.0. Regional correlation indicates that the Eocene detrital sediments were primarily sourced from the Mesozoic magmatic rocks related to the Paleo-Pacific subduction in NW and SW Borneo, with contributions from recycled proximal strata. Pre-Triassic sources are mainly derived from sedimentary and igneous rocks in the Indochina Block. Additionally, the Suwang and Melwai sandstones display features of proximal accumulation. The youngest single-grain zircon age (~46 Ma) in the Suwang Group indicates that the Eocene magmatism in NW Borneo might linke to the tectonic evolution of Proto-South China Sea.