In groundwater quality assessments it is easier and more effective to reduce the number of parameters included in water quality indices. A total of 20 quaternary loose rock pore water and tertiary clastic rock cranny pore water data sets were used for Jilin City, China, as basic data, and 10 water quality parameters were selected for reduction using rough set theory and a statistical analysis of groundwater quality. Results showed that the quality of confined water was better than that of phreatic water in the study area. Confined water was of good quality, and met the permissible limits of the Quality Standards for Groundwater of China, with the exception of NH4+ and F−. For phreatic water, the five parameters of total dissolved solids, NH4+, NO2−, Fe, and F− exceeded the permissible limits, with levels of NH4+ and Fe having a 70% and 40% rate of exceedance, respectively. The results indicated that water evaluation before and after attribute reduction was consistent, which suggests that through rough set theory redundant parameters in indices were eliminated but the accuracy of water quality classification remained effective, while the complexity of the calculation was reduced. Rough set theory provides a convenient and appropriate way to manage large amounts of water quality data.