To understand the impact of Xiaolangdi water and sediment regulation on nitrogen migration, transformation in the lower Yellow River and flux to the sea, the surface water samples were collected from two hydrological stations near the reservoir outlet (Huayuankou, HYK) and the river mouth (Lijin Station, LJ) in six times, including before, during, and after water and sediment regulation in 2023. River flow rates, suspended sediment, total nitrogen (TN), water-phase and suspended-phase nitrate nitrogen (NO3--N), ammonium nitrogen (NH4+-N) concentrations, and nitrate nitrogen isotopes (δ15N-NO3- and δ18O-NO3-) were analyzed. The results showed that the flow rates at HYK and LJ stations rosed to maximum of 4350 and 2860m3·s-1, respectively, the TN concentration decreased due to the release of large amounts of clear water, NO3--N concentrations in the water phase increased, while that in the suspended phase decreased during the water regulation stage. Meanwhile, the δ18O-NO3- value in the water phase decreased, and the concentration of NH4+-N in the suspended phase first increased and then decreased. The results indicated that large amounts of clear water flushed the external nitrogen into the water body, and mineration and nitrification occurred during the water regualtion stage, which caused NO3--N concentration in the water phase to increase. Meanwhile, the scouring flow disturbed riverbed sediments, causing their resuspension and the release of NH4+-N into the water phase. As the water regulation progressed, NH4+-N adsorbed on suspended sediments was further mobilized into the water phase. During the sediment regulation stage, the flow rates at HYK and LJ stations first rose to peaks of 4,010 and 3,480 m3/s, then declined to 591 and 726 m3/s, respectively. The sediment concentrations increased significantly, reaching a maximum of 44.6 and 31.6 g·L-1, respectively during the sediment regulation. Meanwhile, TN, NO3--N and NH4+-N concentrations in the water phase and suspended phase decreased significantly from the pre-sediment to the post-sediment regualtion stage, and NO3--N concentrations at LJ station decreased more than those at HYK station. Compared with the water regulation stage, the δ15N-NO3- in the suspended phase increased during sediment regulation. The δ15N-NO3- values increased and NO3--N concentrations decreased in the suspended phase along the flow direction. This indicated that the coupling effect of nitrification and denitrification occurred during the water and sediment regulation stage. Based on the TN flux calculation at LJ station, the TN flux into the sea was the highest in the post-water regulation stage (1867 t·d-1), followed by pre-sediment regulation stage (1724 t·d-1) and the pre-water regulation stage (1102 t·d-1). The research results will provide a scientific basis for the relationship between water and sediment changes and nitrogen output changes in the lower reaches of the Yellow River.