Abstract: As a key region of the East Asian monsoon system, the South China Sea (SCS) is characterized by high sedimentation rates, allowing its sediments to preserve comprehensive records of both global climate change and regional environmental evolution. This study reconstructs the paleoceanographic environmental evolution of the northwestern SCS since the Last Glacial Maximum (LGM) based on high-resolution diatom records from Core DZ6 on the northwestern continental slope. By analyzing variations in warm-water and coastal diatom assemblages, we infer changes in sea surface temperature (SST) and sea surface salinity (SSS) in the northwestern SCS and explore their responses to East Asian monsoon dynamics. The results indicate that a sharp decline in the relative abundance of the shallow-water species Paralia sulcata at 8～7 cal.kaBP marks the opening of the Qiongzhou Strait. During the LGM and deglaciation, SSS variations in the northwestern SCS were primarily controlled by the East Asian Summer Monsoon, while SST changes were mainly influenced by the East Asian Winter Monsoon. In the Holocene, SSS variations were regulated by multiple factors, whereas SST fluctuations were predominantly driven by the El Niño-Southern Oscillation (ENSO). This study reveals the combined effects of the East Asian monsoon, ENSO, and sea-level change on the paleoceanographic environment of the northwestern SCS, providing critical insights into the future evolution of the SCS’s ocean circulation-climate system under global warming.