Abstract: High-speed water-entry impact load reduction and motion stabilization are the most critical challenges in the development of water-entry vehicles. A truncated cone head equipped with a density-layered crushable cap, which can absorb energy upon breakage, is one effective means of achieving both impact load reduction and motion stability. Model testing was conducted to study the interaction between high-speed water-entry cavity formation and the crushable cap, exploring the effects of density-layered caps on load reduction and motion stability. The study revealed a significant phenomenon of nested cavities in high-speed water-entry vehicles equipped with density-layered caps. The high-density cap created an upper-wide, lower-narrow nested cavity, while the forward-layered cap created an upper-narrow, lower-wide nested cavity. High stress generated by water-entry impact transmits between the two buffering layers of the density-layered cap, resulting in energy loss through collision, thereby dissipating transient impact energy. The cap fracture sequence was observed as follows: first breakage of the forward-layered cap < reverse-layered < low-density < second breakage of the forward-layered cap < high-density. The cap shape also improves motion stability, with the reverse-layered cap achieving stabilization in the shortest time. Additionally, the cap enhances peak speed, with the forward-layered cap condition producing the fastest entry speed, and the high-density cap condition achieving the best load reduction.