Simulated high concentration humic acids (HA) wastewater was treated by Fenton process. The influence of reaction time, initial pH, H2O2 and Fe2+ dosage on the reduction results of COD, TOC, UV254, A400 are presented. The changes of mean oxidation state (eta), A465/A665, the ratio of COD removal by oxidation to that by coagulation (phi) and Zeta potential (zeta) were used to evaluate the roles of oxidation and coagulation in reducing HA during Fenton treatment. The results demonstrate that Fenton's reagent can effectively degrade HA under a wide initial pH range (2.0-5.0), simultaneously the absorbance decrease in 400 nm was higher (from 78.2% to 94.5%) than that in 254 nm (from 75.6% to 88.4%) and the COD removal (from 50.8% to 62.5%) is higher than TOC removal (from 31.2% to 35.1%) in 2 h reaction time. The amount of HA is removed by both oxidation and coagulation. Oxidation played a primary role in removal of HA at the beginning of Fenton reaction. The large molecular weight component of HA appears to be easily degraded and the formations of low molecular persistent organic intermediate compounds are difficult to be mineralized. The COD removal efficiency by oxidation decreases over the ferrous dosage of 0.08 mol/L. Furthermore, the results reveal that HA removal by coagulation was reduced mainly by charge neutralization as well as adsorption bridge building. Results highlight the role of oxidation in controlling the efficiency of COD removal by coagulation, so high COD(oxid) may cause relatively low COD(coag). Meanwhile, ferrous dosage greatly influences COD removal by coagulation at high peroxide dosages over 0.2 mol/L.