On the basis of the finite difference method considering the effect of excavation unloading, one back analysis model of mechanical parameters is established by using orthogonal design theory, neural network and genetical gorithm, and the relative mechanical parameters of the left bank intake slope of Longtan Hydropower Project are back analyzed. The n, the excavation process of construction of the intake slope is simulated by fast Lagrangian analysis of continuance in 3 dimensions (FLAC3D). The deformation pattern and stress state caused by excavation at section 11 of left bank slope are studied, and the slope stability is evaluated comprehensively.