An essential task in evaluating global water resource and pollution problems is to obtain the optimum set of parameters in hydrological models through calibration and validation.For a large-scale watershed,single-site calibration and validation may ignore spatial heterogeneity and may not meet the needs of the entire watershed.The goal of this study is to apply a multi-site calibration and validation of the Soil and Water Assessment Tool (SWAT),using the observed flow data at three monitoring sites within the Baihe watershed of the Miyun Reservoir watershed,China.Our results indicate that the multi-site calibration parameter values are more reasonable than those obtained from single-site calibrations.These results are mainly due to significant differences in the topographic factors over the large-scale area,human activities and climate variability.The multi-site method involves the division of the large watershed into smaller watersheds,and applying the calibrated parameters of the multi-site calibration to the entire watershed.It was anticipated that this case study could provide experience of multi-site calibration in a large-scale basin,and provide a good foundation for the simulation of other pollutants in follow-up work in the Miyun Reservoir watershed and other similar large areas.