The tunnel boring machine (TBM) is typically used in hard-rock tunnel excavation. Owing to the unsatisfactory adaptability of TBM to the surrounding rock, when crossing high-strength and high-wear strata, the TBM can easily cause defects, such as abnormal wear on cutters and overload damage to bearings, thus affecting the construction efficiency and cost. Therefore, high-pressure waterjet technology should be applied to assist in rock breaking for efficient TBM tunneling. In this study, the effects of water pressure, nozzle diameter, and nozzle speed on cutting are investigated via laboratory experiments of cutting hard rock using high-pressure waterjets. The penetration performance of the TBM under different water pressures is investigated via a field industrial penetration test. The results show that high-pressure waterjets are highly efficient for rock breaking and are suitable for industrial applications, as they can accommodate the advancing speed of the TBM and achieve high-efficiency rock breaking. However, during the operation of high-pressure waterjets, the ambient temperature and waterjet temperature in the tunnel increase significantly, which weakens the cooling effect of the cutterhead and decreases the construction efficiency of the TBM. Therefore, temperature control and cooling measures for high-pressure waterjets during their long-term operation must be identified. This study provides a useful reference for the design and construction of high-pressure water-jet-assisted cutterheads for breaking road headers.