A conceptual flow-sedimentation coupled simulation model for large basins was structured by combining the conceptual equations for interrill and rill-gully soil erosion,and its transport formulations which all described in paper 2 with, a conceptual flow simulation model for large basins which was proposed in this paper accordirag to characteristics of runoff yields from excess rainfall and processes of snow accumulation and its melt in arid regions of North China. Two difficult problems,lacldng of recorded precipitation data. and too many observed data to treat in simulating flow and sedimentation of a large basin in arid regions, were dealt relatively well with in the model. And the ununiform of some climatic and geomorphologic factors and the different effects of nunoff resulted respectively from rainfall and snow melt on sediment yields were considered. The model was tested with two large basins, and the results were shown that the model structure are resonable,and the simulation of flow and sedimentation were very satisfactory compared to the observed data.