Abstract: Fenton reagent was employed to treat and decolorize the wastewater of reactive black 5 by online spectrophotometry. The effects of FeSO₄ concentration、H_2O2 concentration、pH、and temperature on the discoloration of RB-5 were investigated. The results showed that there were two steps during Fenton oxidize process, and they all followed pseudo first orderkinetics in the first region and second region. Decolorizing reaction rate constant in the first region (k₁) and removal efficiency were increased by the rise of FeSO4 concentration and temperature. Initial FeSO₄ concentration against k is linear correlation and the reaction activation energy is 3.451 kJ/mol. The optimum initial H_2O2 concentration and pH are 2.118 mmol/L and 3, respectively.