Dry granular flow, a unique sediment gravity flow with plastic rheological properties and laminar flow state, is common in mountainous areas and causes significant damage. Retaining walls are crucial for mitigating debris flow damage, but accurately determining their impact on morphological characteristics is challenging. This paper establishes a numerical simulation model using the discrete element method (DEM) to study the influence of retaining walls on debris flow deposition morphology. Model parameters are calibrated against physical model tests to ensure similarity. This paper will introduce the grain size distribution parameters mu and Dc to represent the content of fine particles and coarse particles, respectively, in order to characterize the granular composition of dry granular flow. Research results show that Dc is closely related to deposition morphology parameters, with longitudinal deposition length and width increasing and maximum deposition thickness decreasing with larger Dc. Retaining wall position significantly affects deposition morphology, with longer and wider deposition as distance increases but thinner deposition. Based on these findings, this paper proposes calculation methods for characteristic parameters under natural conditions and restrained by retaining walls. It reveals the influence mechanism of retaining wall position on deposition morphology and finds that the number of contacts and contact forces between particles and between particles and the deposition plate change dynamically. As the value of sample Dc increases, the reduction rate of the number of force chains, which is influenced by the distance of the retaining wall, also rises. This paper also explores a three-dimensional deposition morphology prediction model, with research results expected to provide theoretical reference for studying movement laws of dry granular flow in mountainous areas and disaster prevention and mitigation.