The formation of flood disaster is restricted directly by the conditions of geology and geomorphology.Flood disaster is caused by floodwater,hut floodwater is not equal to flood disaster.The form of flood disaster is an outer dynamic geological process.Rainstorm is a meteorological phenomenon,flood is hydrological phenomenon,while the development from rainstorm to flood water to flood disaster is a geological process.So geology has important usefulness in the preventing and reducing flood.According to the principle of geology.the strategy of the preventing Yangtze River flood is:(1)the main bank along the river should not be heightened continuously:(2)the condition of geology and geomorphology and the fluvial geological effect caused by Three Gorge Project should he considered when the main bank is reinforced;(3)the old river ways should be used to discharge the flood;(4)combining “returning farmland to lake” with digging silt away in lakes;(5)discharge flood with polder an store flood in polder;(6)open bank and discharge the silt designedly;(7)combining the regional synthesis harnessing with all drainage area synthesis harnessing.