Through sampling measure on paddy field weed Sagittaria trifolia under different nitrogen levels, each component nutrient element (N) reproductive allocation was researched in the seeding stage, the flowering stage and the ripening stage. The results indicated that from seeding stage to ripening stage, each organ total nitrogen content and the whole nitrogen content increased, nitrogen element allocation to root and stem increased, to leaf decreased, to the reproductive organs increased. In flowering stage, nitrogen element allocation pattern showed that leaf> root and stem> flower>scape, and nitrogen element to root and stem increased, to leaf decreased with supplying nitrogen increased and nitrogen element reproductive allocation ratio show that N2>N3>N1>CK. In ripening stage, nitrogen element allocation pattern showed that root and stem> leaf> fruit>scape, the priority of nitrogen element was allocate to reserve organ (root and stem), and nitrogen element allocation to root and stem decreased, to leaf increased with supplying nitrogen increased, and nitrogen element reproductive allocation ratio show that N3>N2>CK>N1. Meanwhile, Sagittatia trifolia gave priority to clone reproduction in low nitrogen supply level (CK and N1), to sexual reproduction in high nitrogen supply level (N2 and N3).