To improve the energy release characteristics of aluminized explosives in underwater explosions， a micro / nano self-assembled composite aluminum powder structure was constructed in this study. The effects of aluminum powder configuration and nano-Al content on the energy release behavior of cyclotetramethylene tetranitramine （HMX）-based aluminized explosives were investigated. Scanning electron microscopy was used to characterize the microstructures of the composite aluminum powders， and underwater explosion tests and heat of explosion measurements were conducted to determine the shock wave energy， bubble energy， total energy， and heat of explosion of the explosive samples. The results show that the self-assembly process promotes the attachment of nano-Al particles onto the surface of micron-sized Al particles， forming a relatively stable micro / nano composite interface. In contrast， the physically mixed samples exhibit a more random distribution of nano-Al particles and more pronounced local agglomeration. The shock wave energies of different aluminized explosives show only slight differences， whereas their bubble energy and total energy vary significantly， indicating that aluminum powder configuration and nano-Al content mainly affect the afterburning energy release during the bubble expansion and pulsation stages. Among all samples， SA-Al-5% exhibits the best overall performance， with a total underwater explosion energy of 8.29 MJ·kg^-1， corresponding to 2.35 times that of an equal mass of TNT. Its heat of explosion reaches 8421 kJ·kg^-1， with a TNT equivalent of 2.04. The enhancement mechanism is attributed to the favorable interfacial contact between an appropriate amount of nano-Al and micron-sized Al particles. The early activation of nano-Al promotes the subsequent combustion of micron-sized Al， enabling the afterburning energy of aluminum to be more effectively coupled into the bubble expansion stage. When the nano-Al content increases to 10%， particle agglomeration， an increased oxide-shell fraction， and mismatched reaction timing weaken the effective energy release. These results indicate that a nano-Al content of 5% is well matched with the self-assembled micro / nano interfacial structure， providing a useful reference for optimizing metallic fuel configurations and enhancing bubble energy in aluminized underwater explosives.