Abstract: The rough set(RS)theory is a new mathematical tool for analyzing the uncertainty and imprecise problems. By using the RS theory in the attribute reduction of the groundwater quality indexes,the significance of the attribute between the groundwater quality indexes and the groundwater quality class,and 2 reducts of the original data are obtained. The discernibility matrix is used to find the reductions. Among the 4 groundwater quality indexes,only 3 indexes are necessary. It is shown that the RS theory is simple,precise and practical for the at ribute reduction of the groundwater quality index through the theo-retical analysis and practical application.