Abstract. Sedimentary produced and transported in mountainous area under extreme rainfall by climate change is a challenged issue in recent years, especially in a watershed scale. The scenario approach with coupled simulation by different models could be one of a solution for further discussion under warming climate. With properly model selection, the simulation of projected rainfall, landslide, and debris flow are integrated by fully connection between models. Moreover, a case in Xindian watershed upstream the capital of Taiwan is chose for studying, and two extreme scenarios in late 20th and late 21st century are selected for comparison on changing climate. With sequent simulation, the chain process and compounded disaster can be considered in our analysis. The potential effects of landslides and debris flows are compared between current and future, and the likely impact in selected watershed are discussed under climate extreme. Result shows the unstable sediment volume would enlarge 29 % in terms of projected extreme event. The river bed may have strong variation by serious debris flow and increase about 10 % elevation in main channel. These findings also highlight the increasing risk in stable water supply, isolated village effect, and other secondary disaster in this watershed. A practical reference could be provided by some critical information in our result for long-term adapted strategies.