<p>Fracture failure is a critical factor triggering rockslides in the deeply incised valleys of the Hengduan Mountains. Clarifying the failure mechanism of valley fractures associated with river downcutting is fundamental for risk assessment in these areas. This paper proposes a systematic framework to explore the failure mechanisms of valley fractures during river downcutting processes. Initially, a proportional elevation stretching algorithm is designed to reconstruct the paleo-geomorphology of valley at different geological periods based on current valley geomorphic features. Subsequently, a valley fracture network is constructed using unmanned aerial vehicle (UAV) photogrammetry and Monte Carlo simulations. Then, a multiscale DFN–DEM (discrete fracture network–discrete element method) approach is used to develop an equivalent geomechanical model of the valley. Finally, a continuous downcutting method is designed to simulate the evolution of valley fractures under non-disturbed and steady river downcutting process. The results indicate that valley fractures predominantly occur in shallow subsurface regions, where rock masses experience significant deformation due to lateral unloading and vertical rebound uplift. Besides, key findings highlight that the orientation of shallow tensile fractures is correlated with the topography, and this correlation becomes increasingly prominent as river downcutting deepens, fundamentally due to the shift in the role of gravity (from fracture-derived restraining force to fracture-derived driving force). The framework effectively overcomes the drawbacks of excavation disturbances in numerical simulations of river downcutting, accurately replicating the evolution of large-scale fractures that govern valley slope stability. This provides valuable guidance for construction projects in the deeply incised canyon regions of the Hengduan Mountains.</p>