A method for controlling regeneration a catalyst by an exhaust gas purification device includes: measuring a temperature of exhaust gas flowing into a first catalyst unit; estimating a NOx amount loaded into the first catalyst unit and a slip amount of NOx of the first catalyst unit by using the temperature and an amount of the exhaust gas of the first catalyst unit; calculating a temperature of a second catalyst unit by using the temperature of the first catalyst unit; and estimating a NOx amount flowing into the second catalyst unit by using at least one of the slip amount of NOx of the first catalyst unit and the temperature of the second catalyst unit.