This study systematically investigated the dynamic variations of nitrogen contamination in coastal aquifers, focusing on the land reclamation area of Qingdao Happy Sea Town. Based on improved and refined nitrogen reaction kinetic equations, a reactive solute transport model under variable density conditions was developed. The model was used to quantitatively analyze the impacts of reclamation length and permeability of reclamation aquifers on the migration and transformation of nitrogen in groundwater. The results indicated that reclamation intensified nitrate pollution in groundwater. The mass concentration of NH⁺₄-N gradually increased, with NH⁺₄-N accumulating in a small range at the bottom of the initial aquifer, and its mass concentration could reach three times of the drinking water standard. As the reclamation length increased, the distribution range of NO-₃-N and NH⁺₄-N expanded. In the initial aquifer, the average mass concentration of NO-₃-N and its increase amplitude rose continuously, and the average mass concentration of NH⁺₄-N exhibited a trend of first increasing and then decreasing. When the permeability of reclamation aquifer was higher than that of the initial aquifer, the permeability of reclamation material had little effect on the average mass concentrations of NO-₃-N and NH⁺₄-N in the initial aquifer under steady state condition. However, when the permeability of reclamation aquifer was lower than that of the initial aquifer, the average mass concentration of NO-₃-N in the initial aquifer increased significantly, while the average mass concentration of NH⁺₄-N decreased significantly.