Experiments were conducted to study the underwater explosion performance of titanium hydride/RDX-based (TiH2/RDX) composite explosive. Cylinder charges with different TiH2 particle sizes and contents were prepared and tested. Explosion parameters like peak overpressure, impulse, shock energy, and bubble energy were analyzed. It was notable that underwater explosion performance of TiH2/RDX composite explosive was promoted by addition of small particle size TiH2 (D-50=0.96m), in which case increasing TiH2 content also showed a favorable effect. The maximum increments of specific initial shock energy, bubble energy, and total energy were 10.5%, 6.4%, and 7.1% respectively. However, with bigger TiH2 particle sizes (D-50=20.78m, D-50=136.74m), the explosion parameters and the TiH2 content showed a negative relationship, which reveals that TiH2 particle size plays an important role in determining the reactivity of TiH2. Meanwhile, the interaction between TiH2 particle size and content was significant.