<p>Waterjet-based rock breaking is widely used in mining and underground engineering, yet conventional cavitating, abrasive, and pulsed waterjets are limited by unstable energy concentration and reduced performance under submerged conditions. A key unresolved challenge is the lack of a predictive approach describing the coupled effects of abrasive loading, jet dynamics, and cavitation on rock fragmentation. To address this gap, a novel abrasive–pulsed–cavitating waterjet (APCJ) is proposed that integrates pulsed loading, cavitation collapse, and abrasive impact within a unified system. The use of a piezoelectric transducer enables controllable pulsation while simplifying system complexity. The coupled interactions among these mechanisms enhance energy transfer and significantly improve rock-breaking performance, achieving up to 80 times the efficiency of pulsed-cavitating jets. Results show that the optimal standoff distance follows a coupled scaling with abrasive concentration and particle size, and that erosion efficiency exhibits a nonlinear dependence on concentration with peak performance at 25–30%. A predictive relationship linking these parameters is established and validated experimentally, providing a quantitative basis for optimizing APCJ operation in submerged rock-breaking applications.</p>