In order to analyze the effect of aluminum fiber contents on the underwater explosion performance of RDX-based explosives, the pressure-time curves of composite explosives with different aluminum fiber contents are measured by underwater explosion experiments. Peak pressure, impulse, shock energy, and bubble energy were obtained by analyzing the curves. The results show that the peak pressures of composite explosives decrease with increasing aluminum fiber contents. The shock impulse of the 30% aluminum fiber composite explosive is the highest in all composite explosives. The effects of the 20% and 40% composite explosives are nearly equal to that of the 30% explosive, and the different values of shock impulse among them do not exceed 5%. The specific shock energy of the 20% aluminum fiber composite explosive is the highest in all composite explosives. The bubble energy and explosion energy of composite explosives increase with increasing aluminum fiber contents.