In order to make the IBIS ( integrated biosphere simulator) model better reflect the spatial heterogeneity of carbon cycling process, GIS technology was introduced, redistribution module of soil water and calculation module of solar radiation received by ground surface were modified, and terrain analysis module was added into IBIS in this study. Based on meteorology, topography, vegetation and soil data, net primary productivity (NPP) of forests was estimated, and the spatial pattern of NPP of forests in 2004 and its response to topographic factors were analyzed by using the improved IBIS model in the Zhangjiagou Catchment, east part of northeastern China. The results showed that the improved IBIS model can be applied to simulate and estimate NPP of catchment forests in the east part of northeastern China. The spatial pattern of NPP of forests generally presented that northeast and northwest area had the high values, and then gradually decreased in the central valley parts in 2004. The annual mean NPP of forests was 375 g/ (m2·year) and its total amount was 541郾72 t/ year in the Zhangjiagou Catchment in 2004. Among the different forest types, the aspen鄄birch forest had the highest annual mean NPP (405 g/ (m2·year)), while the mixed deciduous forest had the lowest one (336 g/ (m2·year)). Among the topographic factors, the altitude and slope had little effect on NPP of forests, while the aspect exerted significant impact on NPP of forests. Furthermore, NPP of forests on sunny slope was 41% higher than that on shady slope.