To improve the energy output characteristics of underwater explosives and clarify the influence law of the aluminum-oxygen ratio （R_Al：O） on the energy output performance of Al-3Li alloy-containing underwater explosives. HMX-based explosives containing pure aluminum （HAl） and Al-3Li alloy （HLi） with R_Al：O ranging from 0.2 to 1.0 were designed and fabricated. The surface morphology， phase composition and thermal decomposition properties of the prepared explosives were characterized via scanning electron microscopy （SEM）， X-ray diffraction （XRD）， and thermogravimetry-differential scanning calorimetry （TG-DSC）. The detonation parameters of the explosives were calculated using EXPLO-5 software， and underwater explosion tests were performed at detonation distances of 0.4， 0.6 m and 0.8 m to determine the shock wave energy （E_s） and bubble energy （E_b） of the two types of underwater explosives. The results indicate that the thermal decomposition process of the underwater explosives includes three successive stages： HMX decomposition， AP decomposition， and intense oxidation of metal powders. Compared with explosives filled with pure aluminum powder， HLi explosives exhibit a 53.81 ℃ advance in exothermic peak temperature and a 26.3% increase in mass gain. At a detonation distance of 0.4 m， the peak shock wave pressure of HLi explosives is 4.05% higher than that of HAl explosives. When the R_Al：O is in the range of 0.2-0.6， the E_s and E_b of HLi explosives are 5%-25% and 2.5%-11.4% higher than those of HAl explosives， respectively. For both explosives， the E_b reaches the maximum value at R_Al：O=0.6， while the total energy attenuates continuously with the increase of R_Al：O. The total energy of HLi explosives is up to 9.1% higher than that of HAl explosives. The energy output structure of underwater explosives changes significantly with the variation of R_Al：O. Specifically， low R_Al：O conditions are dominated by E_s， which accounts for 55.0% of the total energy， whereas moderate and high R_Al：O conditions are dominated by E_b， with a maximum proportion of 66.4%.