To calculate soil erosion based on landunit in a catchment is a new and important method for estimation of soil and water conservation measuresTo achieve such a process based calculation in a catchment or a basin, soil erosion model with both landform factors and up and down stream relations in a catchment need to be developedMeanwhile, the runoff flow networks among landunits, which indicate the relations of runoff flow paths between landunits, are necessaryThe flow networks must be connected with soil erosion models to fit the model principlesIn this paper, the traditional technology of grid based flow network was improved according to loess soil erosion rolesA landunit based runoff and sediment flow network model was developed for simulating the flow process in whole basin, from top of basin to its outletThe flow network among landunit was calculated and three important parameters for soil erosion model were afforded: water and sediment flow order among landunits; the spatial distribution of the conflux amount among landunits; and the slope length of the runoff and sediment passing through each landunitThe soil erosion model was developed according to field observation of natural rainfall and artificial rainfalls with different slope, various land cover and different soil conditionsThe model was divided into three models according to landform positions: flat slope model for areas where the slope of landunit is less than 5 degree; slope model for simulation in whole slope; and gully bottom model for calculation of sediments transfer to the outletThese three models were connected with the landunit flow networks, and the parameters for each landunit such as soil type, slope, elevation, vegetation coverage, cultivating measures, landform position, slope length, etc, were automatically transferred from landunit file to the soil erosion model.