Abstract: Spillway slope deformation can be caused by infiltration of rain and fog. It turns to affect the stability of the slope. It has become one of the important problems of the construction of hydropower stations. This paper is based on the structural features of slope slippage belt,cracks and control joints structure developing in atomized slope in the chute segment of the spillway in one southwest hydropower station. It analysis the boundary conditions and deformation and failure modes of potential instability in the condition of flood discharge atomization after the excavation and supporting of the slope. It examines the geological environment conditions of the slope. It uses the mechanism of the flood discharge atomization role on the rock slope. It finds that the major impact of rain and fog to slope is infiltration of rain and fog along the tension crack surfaces of the slope. The infiltration influences the mechanical properties of the structural plane,thus affects the stability of the slope. The analysis of the deformation characteristics is carried out on structural plane of the different softening coefficient in the slope. The trend of the slope deformation and failure after the infiltration of flood discharge atomizing rain and fog is further analyzed using numerical simulation. The results show that different softening coefficients can result in a larger difference among the downstream side slope rock mass of the spillway 0+55500. If the coefficient is the mid value of 075,the spillway chute slope is less stable,and probably has a shear creep deformation or block failure. So it is necessary to strengthen certain support measures to ensure the safety of the slope during the construction period and the longterm stability in the period of the operation．