Abstract: Existing studies on the stability of internal dump slopes underlain by gently dipping weak layers have primarily focused on the effects of dump spatial morphological parameters or inclined basal surfaces, while the influence of the spatial geometric relationship between the internal dump and the basal surface on slope stability has rarely been examined. Under different horizontal intersection angles between the inclination of the internal dump and that of the basal weak layer, significant differences exist in slope stress distribution, potential landslide modes, and potential sliding-body geometries. Taking an internal dump underlain by a gently dipping weak layer in an open-pit coal mine in Xinjiang as the research background, this study investigated the stability of these internal dump slopes during mine turning in the open-pit mine. The deformation characteristics of slopes under internal dump turning angles of 15°, 30°, 45°, 60°, 75°, and 90° were analyzed, and the potential landslide modes of internal dump slopes underlain by gently dipping weak layers were identified. Using the strength reduction method, the variation pattern of slope stability factor with internal dump turning angle was examined. The results showed that the internal dump turning angle had a highly significant influence on slope stability: the slope stability factor decreased logarithmically with increasing turning angle. Moreover, a larger turning angle led to a stronger influence of the weak layer on the potential sliding body, and the potential landslide mode evolved from a cutting-layer-dominated mode to a bedding-plane-dominated composite sliding mode. With increasing turning angle, the basal dip angle of the potential main slip surface increased linearly, while the internal dump space volume increased quadratically. Engineering practice recommends an internal dump turning angle of 45°-60°; if a larger angle is required, slope-stability enhancement measures must be adopted.