Abstract: Fuel tanks of vehicles are vulnerable to both hydrodynamic ram (HRAM) and fuel explosion when subjected to high-speed projectile impacts. To mitigate this coupled mechanical-activational damage, a novel protection method was proposed, in which load-reducing hollow spheres (LRHS) and explosion-proof spheres (EPS) are filled in a hybrid manner inside the tank. The attenuation effect of these fillers on HRAM was investigated experimentally. A series of comparative tests were conducted using a 25 mm two-stage light gas gun. A 12.7 mm spherical steel projectile with a mass of about 8.45 g was launched at approximately 1100 m/s to impact a fluid-filled container under five different filling conditions : fully water-filled (T1, baseline), 94% water-filled with a free surface (T2), fully water-filled with EPS (T3, about 5% in volume), fully water-filled with 36 preset LRHS (T4, about 1% in volume), and mixed filling with both LRHS and EPS (T5, about 6% in volume). The impact velocity was measured by laser screens, the cavitation evolution inside the container was recorded by a high-speed camera, and the pressure-time histories were recorded by underwater free-field pressure sensors. After the tests, the failure modes of the two fillers were examined, and the maximum deformations of the front and rear panels were obtained by three-dimensional scanning. The results demonstrate that the mixed filling scheme (T5) achieves the most significant attenuation: the peak pressure of the shock wave is reduced by 92.9%, and the maximum deformations of the front and rear panels are decreased by 53.6% and 65.3%, respectively, compared to the baseline. In contrast, the free surface condition (T2) shows negligible effects. The EPS alone (T3) and LRHS alone (T4) also provide considerable attenuation, with peak pressure reductions of 87~90% and 79~85%, respectively. The analysis indicates that the LRHS attenuates the HRAM loading via reflect shock wave and provides expansion space, while the EPS dissipates energy through plastic deformation and restricts cavitation development by compartmentalizing the fluid domain. Their combination yields a synergistic effect. The proposed method provides a promising integrated design for enhancing the survivability of fuel tank against coupled HRAM and explosion threats at a low volumetric penalty.