Abstract: The collapses of both a single cavitation bubble and multiple bubbles are investigated numerically, with special emphasis on the conversions between different forms of energies. The governing equations of the bubble flow are established by considering the viscosities, the surface tension force and the compressibilities of the two fluids. First, the collapse motions of a single bubble is simulated and also validated. We subsequently analyze the evolution of the energies during the collapse and find that the energy transfers from the potential energy to the kinetic energy, then to the pressure wave energy. Strong interactions between bubbles are found during the collapse of the multiple bubbles. The motion of the outer bubbles is obviously faster than the inner bubbles. Due to the energy contribution of the outer bubbles, the collapse of the central bubble leads to a release of high-value wave energy when compared to the collapse of the single bubble. Both for a single bubble and for cube-distributed multiple bubbles, the released pressure wave energy decreases when the stand-off distance is reduced. The conversion rate of wave energy is about 10% at the dimensionless distance of γ=1.5.