In the present paper, we proposed a geological evolution model to analyse the deformation mechanism of a slope in Nujiang river. Detail geology investigation was carried out, we found the lithology within the slope is diversity, the granite was found in the upper portion, while the low portion was granulite, which can be classified as soft rock under in weathering state. In addition, we identified a set of vertical valley parallel fractures concentrated at a deep depths, and this set of fractures were only found within the upper portion of the slope, which indicate that the slope is in the state of deformation. The proposed numerical model take the river incision and rock mass degradation progress into consideration, to simulate the initiation and progressive mechanism. The result suggested that the fractures within the slope was induced by the rock mass degradation due to weathering, and also show that the deformation is initiated in the upper portion of the slope, Importantly, the modelling reveal that compression deformation at the toe of the slope resulting in tensile failure in the upper portion of the slope, demonstrate that the rock mass in the slope toe was the key factor to induce the slope deformation, the river incision is a second causative factor for evolution of the slope deformation.