In-situ monitoring data of a hydropower station located in southeast China revealed constant and rapid deformation in the slope at the diversion tunnel outlet. To find out the deformation scope and characteristics, the in-situ monitoring data was analyzed in detail, and in association with the limit equilibrium analysis and 3D numerical simulation, the slope stability was evaluated. Research results indicated that the slope was stable before excavation; but without effective support, soil layer in the quaternary system would slide toward the excavation face after excavation. Deformation of the side slope would develop with time, which means that the deformation scope would extend to the upper part gradually in the absence of in-time support. Suggestions were given as follows: before designing the slope support measures, firstly, more drilling or physical inspection work are needed to detect the scope and depth of quaternary soil layer; secondly, in-situ tests are needed to check the shear strength of the easily-slide quaternary soil and strongly weathered slate layers.