Disclosed are method and apparatus for controlling a reactor in a reformer including training a first prediction model for predicting properties of feed and a second prediction model for predicting properties of products; predicting the properties of feed being currently supplied to a reactor set in real time by allowing a first prediction unit including the trained first prediction model to receive a current operating condition of the reactor set; predicting the properties of products being produced in the reactor set in real time by allowing a second prediction unit including the trained second prediction model to receive the current operating condition and the predicted properties of feed; calculating amount of temperature fluctuation for each reactor as a control signal for controlling each of the reactors; and controlling an operating temperature of each of the reactors based on the calculated amount of temperature fluctuation.