The Ulan Buh Desert has significant effect for the climate change and hydrologic cycle process in Northwest China. In this paper, evolution of the groundwater in the Ulan Buh Desert was studied based on hydrogeochemical analyses results. A total of 33 representative groundwater samples were collected from different wells to monitor the variation of water chemistry. Based on hydrogeochemical analysis, it is revealed that shallow groundwater chemical type in the Bayan Ulan Mountains piedmont is Cl~--HCO_3~--SO_4~(2-)-Na~+; from Jilantai Lake to Ulan Buh Desert, shallow groundwater evolves from Cl~--HCO_3~--Na~+ type to Cl~--HCO_3~--SO_4~(2-)-Na~+ type, and deep groundwater changes from HCO_3~--Cl~--Na~+ type to HCO_3~--Cl~--Na~+ and then to HCO_3~--Cl~--Ca~(2+). Gibbs plot shows that groundwater chemistry in Ulan Buh Desert is mainly controlled by evaporation and dissolution of many minerals, with slight contribution of precipitation. Relations between major cations and anions and saturation indices show that major sources of ions in the study area are mainly the dissolution of halite, Glauber's salt, albite, calcite, dolomite, gypsum. In addition, chloro alkaline indices and Ca/Na ratio also demonstrate that ion exchange is a major mechanism controlling the composition of groundwater.