## 1 Introduction  
In practical engineering, in addition to the static load, the building structure will also be subjected to dynamic impact loads such as impact, vibration, and explosion. For example, high-rise buildings are always subjected to wind loads and earthquake loads; military protection projects suffer from blasting and impact loads. As a brittle material, ordinary concrete has weak dynamic load resistance. Ultra-high-performance concrete (UHPC) is a kind of building material with ultra-high strength, ultra-high toughness, and ultra-high durability [1], so it is widely used in such fields as long-span bridges, protective engineering [2], anti-seismic structures, and super-high-rise buildings [3; 4]. At present, much research has been conducted on UHPC and improved the mechanical properties of the matrix by adding fiber. Common fibers are steel fiber, carbon fiber, polypropylene fiber, basalt fiber, etc. Compared with other fibers, steel fibers are usually used in most UHPC due to their ease of processing, lower cost, and less water consumption [5]. [PERSON] [6] studied the effect of fiber content on the mechanical properties of UHPC. [PERSON] [7] found that the bending performance of UHPC was significantly improved by the addition of steel fiber through the single-fiber drawing test. [PERSON] [8] used three different large-size steel fibers (straight fiber, hook fiber, and twisted fiber) and micro straight steel fiber, and found that the bond strength and energy consumption of the large-size straight fiber were improved, while those of the hook and twisted fiber were reduced. [PERSON] [9] studied the influence of mixed fibers on the tensile properties of ultra-high-performance hybrid-fiber concrete (UHP-HFRC). The results showed that the macro fiber with the twisted geometry had the best performance, while the smooth long fiber had the worst. Current research mainly improved the performance of the matrix by improving the interfacial bonding property between a single fiber and concrete [10; 11; 12; 13]. The flexural and tensile properties of UHPC are strongly influenced by fiber distribution [14; 15]. However, under the conditions of ordinary pouring technology, the steel fibers inside the UHPC specimen showed a three-dimensional disorderly distribution, and most of the fibers could not play the role of a bridge in the force direction [16]. Therefore, it is necessary to study related technologies to improve the efficiency of fiber resistance to static and dynamic loading. To achieve the steel fiber orientation effect, [PERSON] et al. [17] found that fiber distribution characteristics had a great influence on the flexural strength of steel-fiber-reinforced ultra-high-strength concrete. [PERSON] [18] used the needle extrusion technique to align the cement-embedded short carbon fibers unidirectionally along the direction of motion of the guide nozzle. [PERSON] [19] et al. studied the influence of internal fiber orientation and distribution on the fluidity of UHPC by single-side casting and middle casting of the mold. [PERSON] [20] developed an L-shaped device with narrow horizontal channels to control the flow of fresh UHPC and found that the fiber orientation factor increased. Fiber aligning not only improved the ultimate tensile and flexural strength of UHPC, but also significantly improved the cracking strength and ductility due to the orientation of fibers parallel to the casting direction after fiber aligning [21]. Although many scholars have studied the mechanical properties of concrete after directional molding, they only studied the static mechanical properties, and rarely studied the dynamic impact properties. In this study, the steel-fiber-oriented technology reported in reference [21] was used to prepare the UHPC specimen with different contents of steel fiber (2%, 4%, and 6%). Then, using the split-Hopkinson pressure bar (SHPB), the dynamic mechanical properties of the UHPC specimens with directional distributed fiber were systematically studied.