Abstract：At first，the slope geological conditions as well as dam site geostress and deep fractures in the left bank of Jinping first stage hydropower station have been investigated. Then the evolvement and development of Jinping first stage hydropower station valley are simulated by using distinct element codes UDEC and 3DEC；and stress field，deformation and unloading zone distribution of valley are comprehensively analysed. Based on the simulation results and the deformation of present slope，it is considered that the deep fractures at elevation 1 700– 1 850 m at the left bank of Jinping first stage hydropower station are the yield results due to valley erosion to elevation 1 830–1 730 m at the special geological structures，such as soft upside rock and hard underside rock，the dip direction of rock reverse against slope，wide upside and strait underside of valley and the high regional tectonic stress of Jinping first stage hydropower station valley. From the view of engineering geology，the effect of deep fractures at the left bank on the slope stability is different from tectonic faults，and its influence is limited and local. Be sure not to disturb rock masses under elevation 1 680 m is the best way to keep engineering slope safe and to prevent the deep fractures from developing.
