<p>To reveal the hydrochemical characteristics and controlling mechanisms of surface water in the Qinghai Lake Basin during the frozen period, this study conducted groundwater sample collection in December 2024 in the Qinghai Lake area and major inflowing rivers such as the Buha River, Shaliu River, Quanji River, Hargai River, and Ganzi River. The lake water of Qinghai Lake is alkaline (pH 9.08), with an average TDS of 12,024 mg·L⁻¹. Its ionic composition is dominated by Na⁺, Mg²⁺ (cations) and Cl⁻ (anion), and the hydrochemical type is Cl-Na·Mg, belonging to a typical saltwater lake. The hydrochemical composition of the lake is mainly dominated by evaporation-crystallization processes. The inflowing river water is weakly alkaline (pH 7.19 ~ 7.68), with TDS all below 500 mg·L⁻¹. The ionic composition shows diversity affected by the rock types in the basin, with hydrochemical types including Ca·Na-HCO₃·SO₄·Cl (Buha River), Na-HCO₃ (Quanji River), Na·Ca-HCO₃ (East Shaliu River, Shaliu River, Hargai River), and Ca·Mg-HCO₃·Cl (Ganzi River). These rivers are mainly controlled by rock weathering processes. Among them, the Buha River and Ganzi River are significantly affected by the combined weathering of carbonate rocks and silicate rocks; the Quanji River, Shaliu River, Hargai River, etc., are synergistically affected by evaporation and rock weathering, mainly dominated by the weathering of carbonate rocks and evaporites. This study reveals the uniqueness and formation mechanisms of the surface water hydrochemical characteristics in the Qinghai Lake Basin during the freezing period, providing basic data for understanding the hydrogeochemical connections between inland saltwater lakes and their inflowing rivers, as well as for assessing the evolution of the basin’s ecological environment.</p>