<p>High-altitude alpine regions serve as vital global freshwater reserves, with alluvial groundwater playing a key role in supporting regional water supply and hydrological processes. However, the hydrogeochemical processes associated with alpine alluvial groundwater remain poorly understood owing to the complex climate–hydrogeological conditions (e.g., glacier-snow meltwater-dominated recharge), and this hinders effective implementation of sustainable water-resource-management strategies. Addressing this knowledge gap is essential for protecting such sensitive hydrological systems. This study investigated the hydrogeochemical processes in a high-altitude alpine alluvial fan in northern China, by integrating stable water isotope analyses, hydrogeochemical measurements, and PHREEQC inverse modeling. Results show that water–rock interaction (predominantly rock weathering) governs groundwater hydrochemistry: carbonate and sulfate mineral dissolution supplies Ca²⁺, Mg²⁺, HCO₃⁻, and SO₄²⁻, while Na⁺ is primarily derived from cation exchange and silicate hydrolysis, with minor halite contributions. Gypsum dissolution triggers a common-ion effect, promoting calcite precipitation and dedolomitization, especially along deeper groundwater flow paths. Stable water isotope compositions indicate that glacier-snow meltwater and atmospheric precipitation are the primary groundwater recharge sources, with groundwater evolving from HCO₃–Ca·Mg (dominated by glacier-snow meltwater) to HCO₃·SO₄–Mg·Ca within the alluvial fan. Dissolved CO₂ exerts a key control on mineral dissolution, with increasing pCO₂ and decreasing pH values along flow paths, enhancing Mg²⁺ and SO₄²⁻ enrichment. These findings provide a conceptual framework for deciphering alpine alluvial-groundwater hydrogeochemical evolution and offer practical insights for regional water resource management.</p>