Abstract: To address the short effective distance and low efficiency of pure water jet in slotting for pressure relief and permeability enhancement in coal seams, a viscoelastic water jet rock-breaking technology based on a high-molecular-weight polymer was proposed and investigated. To determine the influence and mechanism of a high-molecular-weight polymer on the jet’s coherence and rock-breaking performance, a systematic study was conducted on the viscoelastic properties of Polyacrylamide (PAM) solutions at various concentrations, the evolution of the jet’s flow field structure, and its cutting and fragmentation performance and mechanisms on coal and sandstone. In the experiments, PAM with a molecular weight of 22 million was used as an additive to prepare viscoelastic working fluids of different concentrations (0.03%−0.15%). First, a rheometer was used to measure rheological parameters such as the storage modulus, loss modulus, apparent viscosity, and relaxation time of the working fluids, and their microscopic micelle structure was observed using a Transmission Electron Microscope (TEM). Second, high-speed photography was employed to capture the jet morphology at different pressures and concentrations, and the jet’s diffusion angle and coherent length were quantitatively analyzed. Finally, cutting experiments were performed on coal and sandstone to compare the cutting depth and kerf morphology of the pure water jet and the PAM viscoelastic jet at 20 and 50 times the nozzle diameter (d). Scanning Electron Microscopy (SEM) was used to reveal the differences in the rock-breaking mechanisms of the two jets at the micro-scale. The results show that the PAM working fluid exhibits significant viscoelasticity and shear-thinning properties. High-concentration (≥0.06%) PAM solutions possess a micelle structure and a distinct elasticity-dominated region. The relaxation time reached a minimum of 0.01 s at a concentration of 0.12%. TEM images revealed that the 0.12% concentration solution formed a uniform and stable network-like micelle structure, achieving a balance between elasticity and fluidity. The addition of PAM significantly improved the jet’s coherence; the diffusion angle and coherent length showed a non-linear relationship of “first increasing, then decreasing” with the increase in PAM concentration. At the optimal concentration of 0.12%, the jet’s diffusion angle was reduced by up to 83.9% and the coherent length was increased by 6.3 times compared to the pure water jet. The viscoelastic jet’s flow field demonstrated a unique structural evolution: a “necking” phenomenon occurred in the coherent section due to the elastic retraction of polymer chains, followed by an entanglement-breakup section induced by high local shear rates. The rock-cutting experiments indicated a substantial enhancement in the rock-breaking capability of the viscoelastic water jet. When cutting coal, the 0.12% PAM jet created a multi-layered fracture structure, whereas the pure water jet only formed a regular kerf. For sandstone, at standoff distances of 30 mm (20d) and 75 mm (50d), the cutting depth of the 0.12% PAM jet increased by 278.4% and 311.2%, respectively, compared to the pure water jet, achieving an optimized effect of increased cutting depth and reduced kerf width. Micro-scale analysis of the rock-breaking mechanism revealed that the pure water jet only caused physical erosion of the mineral surface, leaving the crystal and cementation structures intact. In contrast, the viscoelastic jet was able to induce both intergranular and transgranular fractures, promoting the disintegration of mineral particles and causing deeper damage. Therefore, the addition of PAM to form a non-Newtonian viscoelastic fluid can significantly optimize the water jet’s flow field structure, suppress its atomization and energy dissipation, and thereby substantially enhance its coherence and effective operational distance. This technology shows promising applications in fields such as coal seam slotting, pressure relief, and permeability enhancement.