[Purposes] This paper aims to investigate the working mechanism of ultra-high performance mixture (UHPM)-filled steel tube connections. [Methods] By employing the thread pitch and diameter of steel tubes, as well as the anchorage length of reinforcement bars as variable parameters, 12 groups of reinforcement bar specimens connected by UHPM-filled steel tubes were prepared, with uniaxial tensile tests carried out. [Findings] When the specimen′s reinforcement anchorage length meets the requirement, the steel tube diameter is suitable, threads are incorporated inside the steel tube, and the failure mode of the specimen involves the reinforcement bar being fractured?without being extracted from UHPM. Connections made with UHPM-filled steel tubes are reliable. However, for specimens featuring a smaller steel tube diameter without threads inside, bond-slip failure tends to occur between the steel tube and the grout, which significantly diminishes the tensile properties of the specimen. Additionally, the reinforcement anchorage length is a critical factor influencing the tensile performance of the specimen. When the reinforcement anchorage length is less than seven times its diameter, bond-slip failure?is likely to occur between the reinforcement bar and UHPM, preventing the reinforcement bar from fully developing its tensile strength.[Conclusions] This study lays the groundwork for advancing the application and popularization of UHPM-filled steel tube connections in practical projects.