An eco-hydrological model system (RIP_ N) was constructed to simulate the nitrogen (N) removal by riparian zone in reservoir catchment scale. As a case study, the N removal in Guanting Reservoir riparian zone from March to September in 2007 was estimated. At same time, the field simulated experiment was carried out in Yanqing experimental station, which lies in the northeast part of the catchment. With the experimental data and previous studies, the RIP_ N model was calibrated. RIP_ N model was consisted with two parts, which were soil chemical process modeling and plant growth modeling. Soil chemical processes considered the soil denitrification, nitrification and ammonium volatilization. Plant growth included net primary productivity (NPP) module, plant production allocation module and nutrition uptake module. The research indicated that the correlation coefficient between simulated value and monitored value was larger than 0.5, which proved the effectiveness and reliability of RIP_ N model in catchment scale simulating. The simulated results showed that the N removal loss by riparian zone in Guanting Reservoir catchment from March to September was 5.91 x 10(3) t. The model also identified the N removal functions of different land use. At present land use condition, the bottomland, forest land and grassland contributed positively environmental benefits and removed most of N. In the temporal scale, the N removal from March to September consisted 76.5% of annual removal load. On the contrary, the wetlands just removed 5.9% of N of whole watershed. By comparison, the riparian zone was recognized as critical location for non point source pollution prevention.