Along with the rapid development of towns,the variation of uderlying surface occurs,which obviously influences the rainfall-runoff process.Long-term hydrologic impact analysis (L-THIA),used to simulate the long-term impact on the runoff due to urbanization,is a hydrologic model based on GIS technology.L-THIA is used in the Xitiaoxi River Basin to analyze the effect on runoff with 31 years' precipitation data of the studied watershed,and the relationship between urbanization and rainfall-runoff process is discussed and analyzed.At last,the influence of urban extension on rainfall-runoff process is simulated and predicted, and it provides valuable information for the elementary research of the urbanization's long-term impact on the overland runoff.The changes of land use and land cover,especially the development of towns,add many difficulties to hydrology simulation,which brings newer and greater challenges to urban hydrological models.