A method of determining the stage of geomorphic development is built up in complicated mountainous river basin, which can also be used to regional assessment of the regional severity of collapse-landslide caused by earthquake or heavy rainstorm. Based on the theory of the erosion cycle and the theory of river's developing by stages, the stage of geomorphic development is estimated by combining the Strahler's integral (HI) and the stream-gradient indices (SL indices). Then regional severity of collapse-landslide is estimated by development stage with DEM (digital elevation model) and ArcGIS technique. Both of the change point of HI of branch basins and the breakpoint of SL indices can be the cut-off point of development stage. In the same development stage, HIs are very similar. HI of the whole same development stage can be the representative HI of all the HIs of branch basins. The basins in development stage of late youth and maturity have the severest regional disasters, with HI change from 0.45~0.6.