This work aimed to degrade high-concentration p-nitrophenol (PNP) by Fenton oxidation. We studied various reaction parameters during Fenton oxidation, such as the iron dosage (as Fe2+), the initial concentration and temperature of PNP, and the dosage of hydrogen peroxide (H2O2), especially the influence of temperature on the PNP degradation rate and degree. Under the addition of the same molar ratio of H2O2/Fe2+ and H2O2 dosage according to the theoretical stoichiometry, the PNP degradation rate and the removal rate of total organic carbon (TOC) increased significantly with the increase in the initial PNP concentration. Moreover, the oxidative degradation effect was significantly affected by temperature. The increased reaction temperature not only significantly reduced the Fe2+ dosage, but also greatly promoted the removal rate of chemical oxygen demand (COD) and TOC, and improved the utilization efficiency of H2O2. For example, when the initial concentration of PNP was 4,000 mg·L−1, and the dosage of Fe2+ was 109 mg·L−1 (H2O2/Fe2+ = 200), the removal rates of COD and TOC at 85 °C reached 95% and 71% respectively. Both were higher than the 93% COD removal rate and 44% TOC removal rate when the dosage of Fe2+ was 1,092 mg·L−1 (H2O2/Fe2+ = 20) at room temperature.