Based on the distributed hydrological model SWAT， the hydrological process was simulated with the downstream of the Zhexi reservoir in the Zishui Basin as the study case. Based on pre-processing the data of DEM， land use， soil and weather， the applicability of the model was demonstrated by making parameter calibration with the observed runoff data from 2010 to 2011and validation with the observed runoff data in 2012. The comparison between the simulated and observed monthly runoff show that the correlation coefficient R2 and Nash coefficient of Ens are more than 0.93 and 0.91 in calibration period and validation period respectively， and daily runoff show that correlation coefficient R2 and Nash coefficient of Ens are more than 0.78 and 0.70 in calibration period and validation period respectively. SWAT model has a good applicability for the Zishui basin. It could provide important technical support for water utilization security analysis and environment impact assessment of nuclear power plant.