By taking the Zhangjiachong small watershed in the Three Gorges Reservoir for an example,rainfall-runoff process simulation is conducted to provide references to the researches on soil and water lossand non-point source pollution process and scientific supports to control soil and water loss and non-pointsource pollution,to ensure water security,and to alleviate the growing pressures on ecological environment.Based on the observed data of hydrology and temperature and digital elevation model,TOPMODEL,a halfdistributed hydrological model,is used to simulate daily runoff in the study area.Nash-Sutcliffe coefficient,Root Mean Squared Error（RMSE）and correlation coefficient are taken as objective functions.Simulation results show that the measured values of runoff dynamics are roughly consistent with the simulated values.Simulation accuracy and fitting efficiency in wet season are better than those in dry season and they varygreatly.According to the“ hydrological forecasting standards” SL250-2000,runoff simulation accuracygrade is in C class in wet season and below C class in dry season.TOPMODEL is appropriate for the dailyrunoff simulation in wet season,but not appropriate in dry season.