The Ordos Cretaceous Groundwater Basin, located in an arid-semiarid area in northwestern China, is a large-style groundwater basin. SO42? is one of the major harmful components in groundwater. Dissolved SO42? concentrations, and δ34S-SO42? and δ18O-SO42? in groundwater from 14 boreholes and in gypsum from aquifer were analyzed. Results show that SO42? in shallow groundwaters originates from precipitation, sulfide oxidation, and dissolution of stratum sulphate, with a big range of δ34S values, from ?10.7‰ to 9.2‰, and addition of SO42? in deep groundwater results from dissolution of stratum sulphate, with bigger δ34S values, from 7.8‰ to 18.5‰, compared with those in shallow groundwater. This research also indicates that three types of sulphate are present in the strata, and characterized by high δ34S values and high δ18O values-style, high δ34S values and middle δ18O values-style, middle δ34S values and low δ18O values-style, respectively. The δ34S-SO42? and δ18O-SO42? in groundwater have a good perspective for application in distinguishing different groundwater systems and determining groundwater circulation and evolution in this area.