Abstract: The Early Eocene Climate Optimum (EECO) represents the peak of the early Paleogene greenhouse climate. However, a comprehensive understanding of the terrestrial paleoenvironmental response to the EECO and its implications for organic matter (OM) enrichment remains lacking. We integrated sedimentological, astrochronological, and geochemical data from South China Sea sediments to reconstruct the paleoenvironment and establish the OM enrichment model during the EECO. Astronomical time scales (ATS) for the Lower Wenchang Formation (Lower WC Fm.) in the Kaiping Sag, South China Sea, were established, spanning 55.4 Ma to 43.9 Ma. During 51.5–48.7 Ma, records of astronomical signal (with overlapping cycles of 2.4 Ma, 1.2 Ma, and 405 kyr), stratigraphy (organic-rich mudstone), and paleoclimatic reconstructions (warm and humid climate) provided convincing evidence for the EECO in Kaiping Sag. This study presented the first detailed record of the terrestrial paleoenvironment response to the EECO in the South China Sea, characterized by high terrestrial input, anoxia water conditions, and elevated paleo productivity. A transient pre-warming event before the EECO exhibited a similar paleoenvironmental response, highlighting the sensitivity of terrestrial records. Post-EECO conditions showed a reversal of paleoenvironmental trends observed during the EECO. Pearson correlation analysis reveals that the EECO influenced OM enrichment by regulating paleo productivity and preservation conditions of lake. Elevated atmospheric pCO2 levels and increased terrestrial input promoted algal blooms, thereby enhancing lake productivity. OM preservation was controlled by water column stratification and bottom water anoxia, driven by increased terrestrial input and rising lake levels. Our findings enhance the understanding of feedback mechanisms in terrestrial environments during global warming and provide insights into future climate change predictions.