Characteristics and hydrodynamic mechanism of sediment yield in different topographical units of different scales in Loess hilly-gully region were studied using Qiaogou Watershed in that region as prototype for observation and making use of the observation facilities therein including three hydrological stations and three large-scaled field runoff plots , which located in different topographical units. Results show that the soil erosion modulus displayed a trend of “rising-peaking-declining” with the escalation in spatial scale. At the slope scale, sediment yield was mainly related to depth of runoff. The relationship could be well described by runoff shear stress, with which sediment delivery rate increased. At the watershed scale, sediment yield was related not only to runoff depth but also to peak flood flow. Sediment delivery rate was a power function of the runoff erosion power at the watershed scale.