Abstract: Ion exchange chromatography for preprocessing water samples with nitrate for δ15N and δ18O analysis was further studied and the method of predecessor was improved in this paper. The eluent HCl was defined to 15ml－6M eventually. AgNO3 was transformed into N2 and CO directly by TC－EA avoiding the complicated conversion steps in the past. In this method, AG1-X8(Cl-, 200-400mesh) anion resin produced by Bio-rad3 Corporation had a relatively high affinity for nitrate, which made the sorption efficiency achieve more than 98 percent and the elution recovery achieve more than 94 percent. Moreover, it has a good reproducibility of δ15N and δ18O. The testing accuracy is 0.22‰ for δ15N and 0.4‰ for δ18O, which has reached the level of similar studies abroad and met the requirements of the hydrogeological applications. This method was suited for waters of moderate nitrate concentrations.