Taking the Wanghe River Underground Reservoir in Shandong Province as an example, we used nitrogen and oxygen isotopes to analyze the distribution, sources, and transformation of nitrogen in groundwater. We used the IsoSource software to calculate the contribution ratios of different pollution sources, in order to analyze the influence of the underground reservoir on nitrogen distribution in groundwater. The results show that NO₃^-and TN in groundwater within the underground reservoir had higher concentrations and smaller spatial differences than they did outside of the underground reservoir, and the main nitrogen source was fertilizer in groundwater within the underground reservoir. Denitrification occurred in groundwater within the underground reservoir. The nitrogen in groundwater outside of the underground reservoir was mainly from mariculture and fertilizers. All the results mentioned above indicate that the underground reservoir prevents nitrogen from flowing horizontally, resulting in the accumulation of nitrogen and the enhancement of denitrification of nitrogen in groundwater within the underground reservoir.