Taking Yaoan small watershed in Qingyuan for example,the paper analyzed the causes of mountain flood disaster from the basin rainfall condition, terrain condition, environmental condition of solid material.The result shows that the rainfall in each duration increases obviously by using M-K method. In the future, the possibility of happening heavy rain and mountain flood disaster is greater,and rainfall provide direct power factor for the formation of mountain flood disaster.Using GIS technology,the paper got the result that the valley slope is mainly concentrated in two sides of basin upstream river.The disadvantage gradient potential provide favorable conditions for the formation of mountain flood disaster. The basin rock weathering and unreasonable human activities provide the important environmental conditions for the formation of mountain flood disaster.