The invention relates to a method for treating water through Fenton oxidation of activated molecular oxygen by utilizing ultrasonic catalysis. The method comprises the following specific steps: (1) adding activated carbon powder into waste water, wherein the addition amount of the activated carbon powder is 0.7 weight percent of the weight of the waste water; (2) after the activated carbon powder adsorbs impurities, filtering the waste water, and then injecting the waste water into an ultrasonic reactor; (3) performing ultrasonic crushing reaction, wherein the sound energy density of an ultrasonic wave is 0.2 w / m<3>; (4) after the reaction is performed for 30 minutes, adding transition metal ions, transition metal or a transition metal oxide catalyst into the ultrasonic reactor, introducing air or oxygen, catalyzing the activated molecular oxygen at room temperature to generate a hydroxyl free radical, adjusting the PH value to be 4, and performing stirring; (5) after the stirring is performed for 2 hours, performing sedimentation and solid-liquid separation after the reaction is completed so as to treat the waste water. Sewage is purified by adding the transition metal ions, the transition metal or the transition metal oxide catalyst into the Fenton oxidation under an ultrasonic environment. The method has the advantages of wide oxygen source, low cost, high economy and high reaction rate.