The Qinghai-Tibet Plateau is China's most significant lithium (Li) resource base, characterized by extensive distributions of Li-rich salt lakes and Li-enriched geothermal springs. These two sources are closely linked, forming a coupled exogenous Li mineralization system involving hot springs, rivers and salt lakes. However, the geochemical behavior of lithium during its migration within this system remains inadequately detailed. This study systematically collected water samples from the Xiakangjian hot spring, the Suomei Zangbo river and the Laguo Co salt lake, and analyzed their hydrochemical, trace elemental and Li isotopic compositions. The results show that the dissolved Li in the upper reaches of the Suomei Zangbo primarily originates from silicate weathering, characterized by low Li concentration (7.84 mu g/L) and high delta Li-7 values (+8.21 parts per thousand). Following the discharge from Xiakangjian spring with high Li concentrations (8071.73 mu g/L) and low delta Li-7 (+0.19 parts per thousand), the river exhibits an extraordinary Li enrichment and a negative shift in Li isotopes, with Li concentration increasing by over 200 times reaching 1777.02 mu g/L, and delta Li-7 values decreasing by 8.07 parts per thousand to 0.14 parts per thousand. After passing through the hot spring area, the river's Li concentration stabilizes (1105.49 similar to 1452.33 mu g/L) with a slight increase in Li isotopes (0.12 parts per thousand similar to 0.72 parts per thousand), indicating limited influence from silicate weathering and secondary mineral adsorption on the geochemistry of the Li-rich river water. Consequently, sufficient Li can be stably supplied to the salt lakes via the river, displaying evolutionary characteristics distinct from those of low-Li rivers. The Xiakangjian geothermal spring, Suomei Zangbo and Laguo Co together constitute a continuously operating coupled mineralization system from geological history to the present. The Li-rich hot springs act as the primary endogenous source, the river as the transport pathway, and the terminal lake as the deposition site, with evaporation and concentration driven by the cold, arid climate facilitating the ongoing formation of Li brine in salt lakes. This process completes the migration and enrichment of Li from source to sink, from deep to shallow, achieving a transition from endogenous to exogenous mineralization, and represents a crucial component of the lithium cycle.