The invention provides an electro-Fenton waste water treatment method by adding H2O2. The method comprises the following steps of: regulating a pH (potential of hydrogen) of waste water to be acid to form acid waste water, performing continuous flow decontamination treatment on the acid waste water under a continuous aeration condition to form oxidized waste water, and performing flocculation precipitation treatment on the oxidized waste water, wherein the continuous flow decontamination treatment comprises the steps of adding H2O2 to a source of the acid waste water, taking an iron-containing electrode as an anode and an inert electrode as a cathode, and supplying direct current to the acid waste water added with H2O2 for electro-Fenton reaction decontamination. Experimental results show that a removal rate of a COD (chemical oxygen demand) can reach 57.1%, and a removal rate of TOC (total organic carbon) can reach 41.6% 10min after the method is used for treating reverse osmosis concentrated water of a petrochemical enterprise; and the removal rates of the COD and the TOC are 61.8% and 47.2% respectively 30min after treatment.