Abstract: There are many dykes intersecting in the spillway entrance section and the slope,so the geological conditions are poor,the roof fall occurred because of not timely supporting when excavated,but before the roof falled,and more displacement meter and other monitoring equipment monitored the varying degrees of mutation,geological forecasts also timely predicted the type of surrounding rock and safety prediction according to the excavation condition,this played a forecasting and remindering role in instructing construction and personnel safety.