In recent years, 3D-printed ultra-high-performance concrete (3DP-UHPC), driven by intelligent construction technology, has become a research hotspot in materials science and civil engineering. This is due to its unique combination of the advantages of 3D printing—such as automation, formwork-free construction, and high flexibility—with the superior material properties of UHPC, offering enhanced durability and resilience. Understanding the multiscale relationship between the microstructures and macroscopic mechanical properties of 3DP-UHPC is crucial for uncovering its damage and fracture mechanisms under complex stress states. However, such multiscale mechanisms have not been sufficiently explored, limiting the optimization of printing methods and matrix materials for improved structural performance. This is especially true considering that the integration of UHPC with 3D printing greatly exacerbates the microstructural complexity, intensifying issues such as fiber orientation, interlayer interfaces, and pore networks. These factors play a pivotal role in the macroscopic mechanical anisotropy of 3DP-UHPC, making the unresolved multiscale challenges not only more urgent but also far more complex than conventional cast-in-place UHPC. Therefore, this work utilizes advanced micro X-ray computed tomography (XCT) technique to more precisely capture the evolution of damage and fracture behaviour, specifically associated with the complex microstructures of 3DP-UHPC. The multiscale mechanisms of influencing factors, such as fiber content, printing direction, and different compression directions, on crack formation, propagation, and network development until failure were fully investigated. The research results hold promise to deepen the understanding of the damage and fracture mechanisms of 3DP-UHPC and provide data references for performance-optimized microstructural design.