Abstract: Two parallel permanent fluctuating belt will be formed along the two banks of the reservoir because of the natural fluctuation of water level after the finish of the Three Gorges Dam. Taking the classic geological catastrophe in fluctuating belt of the Three Gorges Reservoir area as a subject of research, the paper analyses and discusses the formation mechanism of landslide and collapse which are the main forms of instabiltiy fracture in fluctuating belt of banks.