Three kinds of cylindrical charges based on RDX with different ratios of aluminum to oxygen were tested through underwater explosion. Attenuation law of shock wave peak pressure versus transmission distance at close-field underwater explosion was resolved by high-speed scanning. The effects of Al/O ratios on initial shock wave peak pressure and attenuation were analyzed. Results show that the initial shock wave peak pressure achieves 18.95 GPa, 13.66 GPa and 8.35 GPa respectively when the Al/O ratios are 0, 0.4 and 0.7. The speed of peak pressure attenuation decreases with the increasing of aluminum content. The factors, such as the time of aluminum beginning to react and the degree of reaction,have remarkable effects on shock wave peak pressure at close-filed and attenuation of peak pressure.