Abstract: Ultra-high performance concrete (UHPC) combines outstanding strength and durability; however, its heavy weight and the difficulty of achieving lightweight designs consistently limit its application in structural load reduction and long-span projects. In order to ensure mechanical performance while achieving weight reduction and optimizing the interface structure, this paper introduces shale ceramsite into the UHPC system and systematically studies the mechanisms of internal curing of ceramsite under different curing conditions and pre-wetting levels on the mechanical properties and microstructure of UHPC. The results indicate a significant negative correlation between the content of ceramsite and the fluidity and apparent density of the mix. The internal curing effect of shale ceramsite can enhance the hydration of the cementitious phase and increase the density of the matrix; with a ceramsite content of 30% and a pre-wet water absorption rate of 4%, the compressive strength improves by 9.75% compared to the control group. Under steam curing conditions, the compressive strength increases by up to 26.2% compared to the group without ceramsite, while the change in flexural strength is relatively minor, yet the bending toughness is significantly enhanced. Microstructural analysis shows that pre-wet ceramsite continuously releases water during the curing process, significantly promoting the generation and refinement of C-(A)-S-H gel, noticeably reducing the width of the interfacial transition zone (ITZ), slowing the spatial gradient of the Ca/Si ratio, suppressing the penetration of interfacial cracks, and significantly improving the bonding density between the matrix and aggregates. These findings reveal the synergistic regulation mechanism of ceramsite characteristics and curing systems on UHPC performance, providing a reliable theoretical basis and parameter optimization strategies for the customized design and engineering application of lightweight UHPC.