Focusing on the conversion of pressure energy and internal energy under viscous dissipation, a heat-fluid-solid coupling method is established to study the flow and stress fields of 100 MPa submerged water jets. Results indicate that pressure energy to internal energy conversion primarily occurs at three locations: the nozzle wall, the potential core edge, and the impact wall, with the most intense conversion occurring at the impact wall. The impact temperature of the jet can reach 200 °C, and the high-temperature region covers an area more than 4 times that of the high-pressure. Thermal stress can especially amplify erosion stress by more than 100% and expand the erosion area by more than 400%. Therefore, it serves as a dominant factor determining the optimal spray distance and jet angle in hard rock (E ≥ 40 GPa). With increased spray distance or jet angle, impact pressure decreases, while the high-temperature zone moves toward the high-pressure region, thus increasing the overlap between the two regions. This extended overlap enhances the temperature-pressure coupling effect, consequently reducing the threshold pressure for jet-breaking rock. Therefore, the maximum erosion stress increases first and then decreases, and an optimal spray distance and jet angle exist. The optimal jet angle, defined by the maximum tensile stress, decreases with the dimensionless spray distance increase, ranging between 0° and 40°. This temperature-pressure coupling reduces rock-breaking threshold pressure by 15%–75% for elastic moduli of 40–80 GPa, with maximum erosion stress peaking at a dimensionless spray distance of 9 and jet angles of 15°–20°. When the overlap region decreases, the area affected by the temperature and pressure fields increases, leading to an increase in the rock-breaking area. It is important to note that reducing the rock-breaking threshold pressure and increasing the rock-breaking area are mutually exclusive objectives. It is necessary to optimize the design of the spray distance and jet angle according to the on-site rock-breaking requirements.