The slope of approach channel for the inlet of Jinping-I Hydropower Station on the right bank is steep and complex. The engineering geological conditions cannot be confirmed accurately before the slope is excavated. Dynamic design of the slope is necessary according to the new geological conditions revealed during the slope excavation. During the initial excavation, the bottom sliding surface g_yj3 is revealed as controlling structural surface of the slope. The method of determining its parameters and the stability calculation of the block are discussed in this paper. When the slope is excavated to 1 860 m, the controlling structural surface g_8sz-1 is revealed. The method of determining its attitude is discussed. In order to avoid the surface g_8sz-1 being exposed in construction, the size of the platform at 1827m is redesigned. Moreover, three-dimensional numerical model is established to analyze the deformation characteristics of the block after the platform is optimized to determine the scope of controlling block. The safety factor of the bottom sliding surface point is used to study the slide mechanism of block and the support layout scheme. Finally, the stability of the supported block is evaluated by inversion analysis of the slope body’s monitoring data. This research provides reference for the dynamic construction design of steep slope in hydropower projects.