Abstract: A dissolved oxygen removal method using an activated carbon fiber catalyst thereof which includes the steps of a reducing agent injection step for injecting a reducing agent into the water, a reactor entrance dissolved oxygen measuring step for measuring a concentration of a dissolved oxygen contained in a water, a water flow rate control step for controlling the supplied amount of the water, an activated carbon fiber catalyst reaction step for removing a dissolved oxygen contained in the water with the reaction between the reducing agent and the dissolved oxygen in an activated carbon fiber reactor in which an activated carbon fiber catalyst is packed, and a control step for controlling the feeding rate of the reducing agent in accordance with the water flow rate and the reactor entrance dissolved oxygen, for thereby extending the life time of the facility and decreasing the operational and maintenance costs by fully removing a dissolved oxygen in water used for an steam generator or cooling system.