Abstract: On the basis of the detailed analysis of the valley geological conditions as well as geostress distribution and deep fractures in the left bank of Baihetan hydropower station, the evolvement and development of the site valley are simulated by using distinct element codes UDEC with denudation method to analysis the formation mechanism of deep fractures in the left bank. Simulation results show that valley stress field distribution presents the law of a typical deep valley with three stress districts of relaxation, increase and unchanged, as well as valley bottom existing high geostress fold.Based on the deep analysis of different parts in the valley slope, different stages of the stress variation and present valley geological features, it is considered that the deep fractures are results of the tensile failure. The outcrops of the consequent dip fault zones after the eroded valley unloads make shear failure further grow. Those shear failures are formed because the local stress concentrations make rock shear failure during the valley development process at the special geological structures such as the dip direction of rocks along slope, the high regional tectonic stress, the asymmetric v-shaped valley and rapid incision of Baihetan hydropower station valley.