Abstract: The Kuitun River Basin is a typical area with high fluoride and arsenic in groundwater in the arid inland areas of northwest China. Due to the shortage of water resources in this area, groundwater is the main water source of agricultural irrigation, industrial and domestic water supply in the Kuitun River Basin. It is of great significance to identify the hydrochemical characteristics, evolvement rules, and formation reasons of groundwater in the Kuitun River Basin for rational exploitation, utilization, protection and management of the groundwater resources in this basin, and to ensure the sustainable development of society and economy as well as ecological security of the basin. In order to assess the groundwater quality and the variation trend in the plain area of the Kuitun River Basin, 316 sets of groundwater samples were collected from 2017 to 2019. Descriptive statistical analysis, spatial analysis, Piper plots, Gibbs plots, ion ratios method, saturation index method, etc., were used to analyze the spatial distribution, hydrochemical characteristics, evolvement law and the cause of the groundwater chemistry. The results showed that: (1) The hydrochemical composition of the groundwater has great dispersion and fluctuation in spatial distribution. The chemical type of phreatic water in the pebbly plain area is HCO₃·SO₄-Na·Ca. The type of phreatic water in the fine soil plain area is SO₄·Cl-Na·Ca, and the type of confined water is HCO₃-Na·Ca, HCO₃·SO₄·Cl-Na. (2) The chemical ions in groundwater mainly come from rock leaching, evaporation, and concentration. Among them, K⁺ and Na⁺ were mainly derived from rock salt dissolution, SO₄^2- and Cl^- were mainly derived from evaporation and dissolution, and Mg²⁺ and Ca²⁺ were partly derived from evaporation and dissolution and partly derived from dissolution of silicate and carbonate. CaCO₃ and CaMg(CO₃)₂ were saturated in phreatic water in fine soil plain area and unsaturated in confined water in gravel plain area. However, CaSO₄ was unsaturated in both phreatic water and confined. (3) Cationic alternative adsorption occurred in groundwater at different levels in the study area, in order from the strong to the weak: confined water in fine soil plain area > phreatic water in fine soil plain area > phreatic water in gravel plain area. The study shows that the ion concentration in the study area increases gradually from south to north, and evaporation and rock dissolution are the main sources of ions.