Abstract: Addressing the challenge of treating highly toxic and refractory phenolic compounds (e.g., phenol and its derivatives) in wastewater, this study employed phenol as a representative organic pollutant and established a PDWR–H₂O₂–OA photo-Fenton catalytic activation system. Here, pyrolytic drilling waste residue (PDWR) served as the catalyst, H₂O₂ as the oxidant, and oxalic acid (OA) as the chelating agent. The system was designed to investigate the activation mechanism and photocatalytic performance of PDWR toward H₂O₂ under natural sunlight irradiation. Under solar radiation, the PDWR-based system achieved a 7.51-fold increase in H₂O₂ utilization efficiency. The degradation rate of phenol reached 2.2 mg/g/h, with a mineralization rate approaching 100%. Moreover, PDWR exhibited excellent stability: after five consecutive cycles, the cumulative decline in catalytic efficiency was less than 4.00%, while the phenol degradation rate remained above 77.21%. EPR characterization revealed that the primary active species in the PDWR–H₂O₂–OA system was the ·OH radical, accounting for 84.24% of the reactive oxygen species. SEM analysis indicated that the surface morphology of PDWR transformed from a disordered needle-like structure before the photo-Fenton reaction into an ordered block-like structure afterward. BET measurements showed that the pore structure of PDWR was dominated by macropores prior to the reaction, which shifted to a microstructure primarily composed of micropores following the reaction. Furthermore, PL spectroscopy, photocurrent response, UV-Vis diffuse reflectance, and Mott–Schottky measurements demonstrated that the photogenerated charge carriers in PDWR possessed a low recombination rate upon visible-light excitation. These results collectively confirm that PDWR exhibits favorable photocatalytic performance and holds promising potential for practical environmental applications.