Abstract: The Yuncheng Salt Lake, nestled in the Fen-Wei graben system's heart, stands as not only China's but also the world's earliest known salt lake. Despite its historic significance, fundamental geological background of the lake remains underexplored, with a lack of consensus among researchers. This study delves into three pivotal aspects: the process of geomorphic change in Cenozoic era and the formation of Fen-Wei graben system, the Yellow River's historic course through the Sanmenxia Gorge and the resultant disappearance of the ancient Sanmen Lake, and the Fenhe River's diversion from the Yuncheng Basin and the birth of the Yuncheng Salt Lake. Our findings suggest that evolution of the lake began in the Paleogene to mid-Neogene period, influenced by the coastal Pacific domain and the Tibetan Plateau. Around 700, 000 years ago, a significant geological shift occurred: the Yellow River breached the Sanmenxia Gorge, flowing eastward to the sea. This event marked the end of the ancient Sanmen Lake and the formation of Yuncheng Salt Lake. Approximately 70, 000 years ago, during the middle Late Pleistocene, the Fenhe River rerouted, leaving the Yuncheng Basin. This change transformed the lake from an open to a closed system, solidifying its current form. The ongoing tectonic activity along the northern fringes of the Zhongtiao Mountains further sculpted the landscape, creating sedimentary depressions that amassed saline minerals, eventually giving rise to a natural salt lake through prolonged cycles of precipitation and evaporation. Salt of the lake primarily originates from deep subterranean salt formations. The primary fault controlling the lake acts as the main conduit for these salts, while the marine high magnesium dolomite from the Longjiayuan Formation in the northern edge of Zhongtiao Mountain supplies magnesium ions. These research will offer a crucial geological foundation for the ecological preservation and sustainable development of Yuncheng.