Shigaze city is situated in the southwestern Tibetan Plateau and is the second largest city in the Tibet Autonomous Region. Groundwater is the major source of domestic and drinking water for urban inhabitants. In this study, the major ion chemistry and a water quality assessment of groundwater were studied using geochemical methods and fuzzy comprehensive assessment. Groundwater was classified as slightly alkaline soft and hard freshwater, and the influence of anthropogenic activities on groundwater was relatively weak. The dominant cations and anions were Ca2+ and Mg2+ and HCO3− and SO42−, respectively. Overall, the mean concentrations of major ions in groundwater increase gradually over time, except for NO3−; however, the mean value of pH decreases over time. Most groundwater samples belong to the type of HCO3-Ca, and the groundwater has a trend of evolution from HCO3-Ca to the mixed type. Rock weathering was the main hydrogeochemical process controlling groundwater hydrochemistry, and the dissolution of carbonate and silicate minerals were the primary contributors to the formation of the major ion chemistry of groundwater. Major ions of groundwater in the urban area of Shigaze are below the standard limits, and the groundwater is excellent for drinking according to the fuzzy comprehensive assessment.