Due to the land use changes the natural hydrological circle process and the quality and quantity of water resources are changed greatly, which will influence the development of socio-economy and the quality of the ecological system to a certainty. A research on effect of land use changes on hydrological system, especially on water balance and flood protection is not only of significant academically, but also applicable with regard to different fields relating to sustainable development of the society and sustainable use of resources and environment conservation. The paper mainly study on development of the regional digital hydrological model and predictions of impact of land use changes on the hydrological system by means of a case study of Qinhuai river catchment within Jiangning county. The digital hydrological model is developed, which can describe spatial-temporal changes of the hydrological circle and conveniently output the spatial distribution of hydrological variable and state. It can simulate the impact of the land surface characteristics on the hydrological process too. Its partial parameters can be determined by the geographic information and land surface characteristics and the model calibrating is less dependent on the historic data. So the model can be applied in region lack of hydrological data and can simulate land use changes scenarios with specified representative parameters. Several land use changes scenarios of study area are defined and the scenarios are simulated by the digital model. The results show that the model can reflect the impact of different land use changes on the hydrological system, especially on water balance and flood protection within the catchment quantitatively and qualitatively.