A snail-macrophyte simulation system was built and isotope tracer technique was adopted to study the environmental fate of nitrogen in snail-macrophyte purification system, the results showed that: Vallisneria spiralis increased its wet weight by 580% and its number by 6.6 ramets, moreover, Vallisneria spiralis absorbed 1.07% 15N by the roots and 7.74% by stems and leaves, while Bellamya only absorbed 0.06%. And 5.73% 15N was retained in the sediment. Through analyzing of the results, it indicated that: in such simulation system, sediment was the main nutrition source for the growth of Vallisneria spiralis, which absorbed only few dissolved nitrogen from water; ammonium nitrogen in water was transformed mainly in the sediment-water interface, most of which was absorbed by Vallisneria spiralis, a small amount was removed through nitrification and denitrification, and the rest was kept by sediment; Vallisneria spiralis was final vector for removing nitrogen in the system, and Bellamya could also boost the growth of Vallisneria spiralis and strengthen the processes of nitrification and denitrification, thus promoting the nitrogen removal from the system indirectly. So, during the period of culture, rational allocation of snail-macrophyte structure in different stages plays an important role in controlling water quality in ponds.