Flood　forecasting　is　usually　based　on　the　simulation　of　rainfall-runoff　models.　In　this　study,　based　on　the　saturation　excess　runoff　mechanism,　a　distributed　three-source　runoff　model　was　developed　for　rainfall-runoff　simulation　in　mountainous　areas.　Firstly,　a　watershed　was　delineated　into　several　hillslope　units　and　a　digital　terrain　analysis　was　conducted　to　extract　the　topographic　features　from　the　DEM　dataset,　the　discretization　of　the　parameters　was　realized　by　the　digitization　of　the　watershed　at　the　same　time　and　providing　the　necessary　support　for　the　application　of　the　model.　Secondly,　the　runoff　yield　in　the　slope　surface　unit　was　calculated　by　using　the　flow　continuity　equation　and　Darcy　formula.　Taking　the　simplified　kinematic　wave　equation　of　Saint-Venant　equations　being　as　the　governing　equation,　the　approximate　partial　differential　equation　of　slope　surface　flow　was　derived,　and　the　confluence　of　each　slope　surface　unit　was　calculated　with　characteristic　line　method.　Storm　runoff　can　then　be　simulated　from　the　unsaturated　lateral　flow　to　the　saturated　overland　flow　on　the　hillslope　unit　of　the　proposed　distributed　runoff　model.　Moreover,　outflows　from　the　hillslope　units　were　confluent　and　routed　by　using　an　improved　Muskingum-Cunge　method　in　the　main　channel.　To　test　the　applicability　of　the　model,　the　simulations　by　using　the　hydrological　records　of　Goodwin　Creek　experimental　watershed　in　USA　were　assessed　by　the　deterministic　coefficient,　correlation　coefficient.　The　results　showed　that　the　proposed　distributed　runoff　model　can　afford　precise　rainfall-runoff　simulations　and　the　accuracy　can　meet　the　requirement　of　forecasting.　In　general,　the　developed　model　can　extend　the　applicability　for　rainfall-runoff　routing　from　small　hillslopes　to　mid-size　or　larger　watersheds.　©　2019,　Editorial　Department　of　Advanced　Engineering　Sciences.　All　right　reserved.