Abstract: The Ediacaran-Cambrian transition is a key stage of marine environmental fluctuation and biological evolution in geological history. However, the interrelationships among volcanic/hydrothermal activity, seawater salinity changes, and the Cambrian Explosion in shallow-water areas along the southeastern margin of the Nanhua Basin remain unclear. Drilling samples of the Lower Cambrian from the margin of the paleo-uplift in the central Hunan as research objects were analyzed using scanning electron microscopy, inorganic geochemistry, and silicon isotope techniques to elucidate the enrichment carriers and genesis of mercury in sedimentary rocks, the spatio-temporal distribution of seawater salinity, and the sources of siliceous material. Significant mercury anomalies were observed in the basal Cambrian shales surrounding the paleo-uplift, with mercury primarily enriched in organic matter, serving as an effective indicator of volcanic/hydrothermal fluid input. Volcanic/hydrothermal activity was relatively active during the transition from Cambrian Stage 2 to Stage 3 (about 526 to 521 Ma) and gradually declined in the late Stage 3 (about 518 Ma). Water salinity around the paleo-uplift was relatively high, while the deep-water area of central Hunan showed characteristics of freshwater to brackish environments. This difference may be related to the connectivity with the open sea and the degree of water retention. In the early Cambrian, the area around the paleo-uplift experienced a saline environment, which gradually evolved into brackish to freshwater conditions as water retention increased. Additionally, shales of the Niutitang Formation around the paleo-uplift contained abundant siliceous material. In the lower section, silica was mainly biogenic with minor terrigenous and volcanic origins, while in the upper section, silica primarily sourced from a mixture of silica-rich seawater and hydrothermal input. Episodic volcanic/hydrothermal activities and paleogeographic patterns during the Early Cambrian in the shallow-water areas along the southeastern margin of the Nanhua Basin jointly drove salinity stratification and multi-source silica supply. These findings reveal the differential evolution between shallow- and deep-water basins and provide key geochemical constraints for understanding marine environmental fluctuations in the Nanhua Basin.