Abstract: The structural stability of waste dumps is critically important to the safety of mine production. Taking the mixed tailings-waste rock dump of a specific mine as the research object, this study systematically analyzed the particle gradation characteristics and mechanical response laws of the mixed dump based on field investigation and laboratory tests. Through screening and density tests, the particle size distribution and fine particle enrichment characteristics of the mixed dump in different height sections were revealed. Triaxial tests were conducted to obtain the strength parameters of mixed dump with different mass ratios, and the influence trend of the dump ratio of mixed dump on cohesion and internal friction angle was clarified. Further combined with PFC discrete element numerical simulation, three types of typical bearing skeleton structures were identified, and a structural identification criterion with volume ratio as the boundary was proposed. The research results show that the skeleton structure of mixed dump can be categorized into three types: single skeleton dominated by waste rock (Type Ⅰ), waste rock-dominated skeleton with tailings participating in stress transmission (Type Ⅱ), and co-bearing skeleton formed by waste rock and tailings (Type Ⅲ). When the volume ratio of waste rock to tailings is in the range of 5.0∶1 to 5.7∶1, the mixed dump transforms from an interstitial structure to a skeleton structure, and its strength parameters increase significantly.Reasonably increasing the waste rock proportion is conducive to the formation of a load-bearing skeleton dominated by waste rock particles, thereby reducing the risk of internal stress concentration in mixed dump. This finding can provide a reference for the structural optimization and safety stability evaluation of mixed dump systems in mine waste dumps.