In this study, an extended Fenton process was used to improve biodegradability of the waste drilling mud containing bio-refractory polymers. Variation of biodegradability and organics with different molecular weights with the oxidation time were investigated during the Fenton oxidation process. Although the residual total organic carbon (TOC) arrived at a stable level soon after oxidation reaction, organics with the lower molecular weight increased and its biodegradability was improved significantly in the extended oxidation process, which originated from decomposition of residual H2O2 catalyzed by transformation of the Fe3+/Fe2+ and organoradicals. Under the conditions that follow: pH 3.0, H2O2 500 mg L−1, Fe2+ 250 mg L−1, oxidation time 120 min, further TOC removal of 35.9% and biochemical oxygen demand and total organic carbon (BOD/TOC) ratio of 0.83 was achieved. At the biological test, a substantial increase in TOC degradation by biological treatment with extension of Fenton oxidation time was observed. Finally, more than 90% biological removal of the TOC was achieved for the 120 min oxidation treatment. The experimental results highlight that an extended process can be adopted to improve the biodegradability of wastewater by utilization of the slow reaction of hydrogen peroxide with Fe3+ and organoradicals.