Abstract: During high-speed water entry, the change of medium subjects the vehicle to transient strong impact loads, and the variation of forces leads to changes in motion stability. This paper investigates the load-reduction characteristics of a multistage load reducer during high-speed water entry of a vehicle. The effects of the reducer on impact loads, flow field evolution, and ballistic characteristics are explored through experiments. High-precision sensors and high-speed camera technology are used to record the motion characteristics of the vehicle, and comparative experimental data between the vehicle without the multistage load reducer and that with the reducer are analyzed. The experimental results show that after water entry, both vehicles experience the stages of water impact, cavity evolution, vehicle deflection, and wetted navigation. The cavity evolution process consists of cavity expansion stage, cavity closure stage, and cavity secondary expansion stage. With the installation of the multistage load reducer, less energy is consumed during water entry; the water splash angle decreases from 42° to 33°, and the splash distance is reduced at different time instants. The velocity at 50 ms after water entry increases from 60 m/s to 80 m/s; the cavity becomes more slender, with its maximum diameter decreasing from 4.74D to 2.77D, a reduction of approximately 41.6%, while the cavity closure point depth increases from 4.06L to 4.90L; the peak impact acceleration decreases from 11359.15g to 2067.63g, a reduction of 81.8%; the cavity asymmetry increases from 0.15 to 0.38 at 20 ms; the peak axial acceleration upon water contact decreases from 519.6g to 442.5g, a reduction of 14.86%, and during the 0–20 ms stage, the overall mean axial acceleration decreases by 35.14%, while during the 20–150 ms stage it decreases by only 7.87%; the overall mean radial acceleration increases by 107.4% during the 0–50 ms stage and by 16.7% during the 50–300 ms stage, with the peak radial acceleration increasing from 63.85g to 125.98g, an increase of 97.3%; the peak pitch angle deflection increases from 28° to 55°, and the deflection initiation position advances from 2.87L to 2.51L. The multistage load reducer effectively reduces the axial impact load through a stage-by-stage synergistic mechanism of "cavitator reducing contact area – aluminum foam compression energy absorption – spring elastic buffering". However, the slender and unstable cavity exacerbates shoulder disturbances and flow field asymmetry, leading to earlier and more severe vehicle deflection. A trade-off exists between load-reduction design and ballistic stability. Future research can focus on optimizing the cavitator cone angle, adjusting the aluminum foam density and spring stiffness, or introducing active control structures to improve ballistic stability while maintaining the load-reduction effect.