The synergistic reaction between boron and aluminum particles can ameliorate the ignition and energy release performance of boron components， thereby increasing the explosive energy of energetic charges. To optimize the structural combination and mass ratio of boron-aluminum mixed particles， a double-layer charge configuration consisting of an inner main explosive charge and an outer fuel layer was proposed in this work. The influences of particle size and mass ratio on the explosion performance of charges were systematically investigated via underwater explosion experiments. The experimental results show that when the outer fuel layer is composed solely of flake aluminum powder， micron boron powder or nano boron powder， the peak shock wave pressure of charges measured at a distance of 3 m is comparable， ranging from 10 to 10.2 MPa. Among these single-component systems， the flake aluminum powder exhibits a remarkable enhancement in explosion impulse. For boron-aluminum mixed particles with a mass ratio of 1∶1， the explosion impulse， shock wave energy and bubble energy of the charge are superior to those of pure micron boron powder and pure nano boron powder systems， while inferior to the flake aluminum powder system. The mixed system achieves the comprehensive optimal explosion performance at a boron-aluminum mass ratio of 7∶3. For the composite of micron boron powder and flake aluminum powder under this ratio， the peak shock wave pressure， explosion impulse and shock wave energy are increased by 17.6%， 15.8% and 28.7% respectively compared with the pure flake aluminum powder system， and its total energy accounts for 83.3% of that of pure flake aluminum powder. Further analysis reveals that the particle size of boron powder exerts a negligible effect on the energy distribution proportion of the system. This study demonstrates that rational gradation design of boron-aluminum particles can effectively improve the shock wave performance of energetic charges in underwater explosion and optimize the energy distribution structure.