The migration and transformation of nitrogen in a seawater-constructed wetland treating Paralichthys olivaceus mariculture tail water was investigated using stable isotope technology. The study employed the mass balance method to quantify the contribution of different nitrogen removal pathways to overall nitrogen removal. showed that the compound vertical flow constructed wetland had a significant effect on nitrate nitrogen treatment, with a nitrogen removal rate of (92.81±1.21)% after 21 d of operation. The δ¹⁵N value of the coal residue layer was the lowest (203.58±2.87)‰, while the coral stone layer had the highest δ¹⁵N value (303.66±2.22)‰. Plants exhibited the best capacity for nitrogen uptake, with an average nitrogen content of (2.68±0.38)% and an average absolute abundance of (105.61±14.65) × 10^–3 mg/g. Microbial transformation was found to be the main nitrogen removal pathway, accounting for 60.77% of the nitrogen removal. This study provides effective data support for applying stable isotope technology to the construction of seawater-constructed wetlands and clarifies the nitrogen removal process.