Abstract: Returning farmland to forest is the principal policy presently for improving eco-environment and controlling soil and water losses in the loess hilly-gully region. In order to provide the foundation for the policies to be usefully put in practice, the quantitative relationship between crop yield and topographic conditions was analyzed according to WIN-YIELD software, the meteorologic data of Yan'an weather station in 2002 and the physiognorny, soil, land use in Yangou Basin. The changes of crop yields such as corn, potato, broomcorn, soja and pea with terrain altitude, topographic slope and slope aspect were simulated. The results show that topographic slope gradient has an important influence on crop yield. The bigger the slope gradient, the lower the crop yield. And the impact of terrain altitude on crop yield was slight except for the yield of potatoes, the impacts of slope aspect was slight.