Abstract: Large-scale volcanic activity is recognized as one of the critical drivers of global climate and environmental changes, as well as biodiversity fluctuations. It can enhance marine surface primary productivity and promote the development of euxinic bottom water conditions, thus facilitating organic-rich sedimentation. Across the Ordovician-Silurian transition, intense global volcanic activities coincided with the widespread deposition of black shales and the development of organic-rich intervals characterized by extraordinarily high total organic matter (TOC ≥ 3.0%). Based on more than 800 samples, including TOC, major and trace elements, mercury (Hg) concentrations and Hg isotopes, collected from over 20 typical wells and sections of the Wufeng and Lungmachi formations in South China, this study discusses the occurrence and identification features of visible volcanic ash layers and highlights the application of major and trace elements, Hg concentrations and isotopes to identify cryptotephra. Furthermore, the impact of volcanic activity on marine environmental changes and the formation of extraordinarily high organic matter deposits were investigated. The results suggest that: (1) the distribution of visible volcanic ash and cryptotephra provides a robust basis for reconstructing volcanic history of this period. (2) Elevated phosphorus inputs from volcanic activity enhanced marine surface primary productivity and expanded the extent of anoxic bottom water. (3) The anoxic conditions promoted phosphorus recycling, thereby sustaining high primary productivity. (4) Enhanced volcanic weathering increased sulfate input, which could further promote euxinic conditions. Sustained volcanic activities and their associated climate and environmental effects contributed to the high primary productivity and euxinic conditions, ultimately leading to the extraordinarily high organic matter accumulation in the Wufeng-Lungmachi shale.