Abstract: Groundwater is an important source of recharge to the Yellow River in the area of the Maqu Plateau, but little is known about the groundwater hydrochemical characteristics and the formation mechanism. Phreatic water samples, surface water samples and sediment samples were collected from the study area to investigate their hydrochemical and isotopic characteristics and hydrogeochemical processes. The results show that river water and phreatic water samples have low salinity with Ca²⁺ and \rmHCO_3^- being the main constituents (TDS of 72−195 mg/L and 207−459 mg/L, respectively). Concentration of arsenic (As) in water samples range from 0.46 μg/L to 17.7 μg/L. The plot of hydrogen and oxygen isotopes shows that the phreatic water and surface water are mainly recharged from local meteoric water. The river water is relatively enriched in ¹⁸O and D relative to the phreatic water. The hydrochemistry of the river water are mainly influenced by evaporation, while the phreatic water is mainly influenced by rock weathering involving dissolution of carbonates. Saturation index of dolomite (SI_dolomite) in 68% phreatic water samples is below zero, indicating that the phreatic water is unsaturated with respect to dolomite. As in the groundwater are possibly the result of reductive dissolution of iron-manganese oxide minerals in the unconfined aquifer sediments, which may be sourced from adsorbed As forms in the iron-manganese oxide minerals. This study is helpful in revealing the source of shallow groundwater and genesis mechanisms of groundwater chemicals in the upper stream of the Yellow River.