We investigated nitrogen (N) and phosphorus (P) budgets in different grass carp (Ctenopharyngodon idellus) culture systems within a land-based enclosure. The experiment began in May and ended in October in 2011. Samples were collected periodically, and the enclosed area was 7 m × 7 m. N and P inputs and outputs were evaluated in grass carp monoculture(G), grass carp and silver carp (Hypophythalmichthys molitrix) polyculture (GS), grass carp and common carp(Cyprinus carpio) polyculture(GC), and grass carp, silver carp, and common carp (GSC1 and GSC2) sys- tems. N and P accumulation in water and sediment were analyzed. The results show that fish feed was the main N and P input source in all culture systems (85.54%–93.38% and 82.60%–84.26%). Other sources were the stocked animals, precipitation, and the initial water used. The N output sources in the different culture systems were the stocked animals, sediment accumulation, water accumulation, and absorption and volatilization by the enclosure cloth, which accounted for 62.80%–77.15%, 15.19%–27.60%, 5.04%–7.71%, 1.54%–2.14%, and 0.22%–0.30%, respectively. The P output sources were accumulation in sediment, accumulation in water, culture animals at harvest, and absorption by the enclo- sure cloth, which accounted for 76.46%–80.04%, 13.04%–15.14%, 4.09–9.79%, and 0.71%–1.61%, respectively. Ni- trogen accumulation in sediment from the GSC1 and GSC2 system was significantly lower than that in the G, GS, and GC systems (P<0.05), whereas P accumulation in sediment from the GSC1 and GSC2 systems was significantly lower than that in the G and GS systems (P<0.05). N and P accumulation in water from the GSC2 system was significantly lower than that in the G system (P<0.05). N and P utilization by the GSC2 culture system was significantly higher than that by the G, GC, or GSC1 systems (P<0.05). These results indicate that the GSC2 system (grass carp: 0.38 ind/m2, silver carp: 0.69 ind/m2, and common carp: 0.55 ind/m2) effectively reduced N and P accumulation and enhanced N and P utilization. Thus, the GSC2 system was an effective and clean grass carp polyculture mode.