Based on field investigations, combined with remote sensing images, relevant lacustrine strata and their chronologies, as well as historical literature, a comprehensive analysis was conducted on the scope, extinction time, and causes of the ancient Chenchuan Lake on the southeastern margin of the Mu Us Desert. The research shows that: (1) The actual area of the ancient lake was wider than the previously thought 100 km2, reaching 130 km2. (2) The extinction of the ancient lake occurred around 300 AD, and the waxing and waning of the ancient lake corresponded to the rising and falling trends of temperatures in China and the Northern Hemisphere. (3) Although the temperatures in China and the Northern Hemisphere have increased several times since 300 AD, the ancient lake has never been restored. The reason is related to the transformation of the neotectonic movement from subsidence to uplift in the Salawusu River Valley since then. Moreover, against the backdrop of neotectonic uplift since 300 AD, the "artificial drainage system" caused by human activities" transformation and destruction of the Holocene lacustrine deposits has also accelerated the loss of surface water within the original ancient lake area, preventing the ancient lake from being restored.