A system for purifying an exhaust gas may include a lean NOx trap (LNT) catalyst adapted to absorb nitrogen oxides contained in the exhaust gas at a lean atmosphere, release the absorbed nitrogen oxides at a rich atmosphere, and reduce or slip the released nitrogen oxides according to a temperature thereof; a particulate filter adapted to trap particulate matters contained in the exhaust gas and regenerate the trapped particulate matters by using the nitrogen oxides slipped from the LNT catalyst; and a controller adapted to selectively create the rich atmosphere when the temperature of the LNT catalyst is higher than or equal to a first predetermined temperature or a temperature of the particulate filter is higher than or equal to a second predetermined temperature. A method for controlling the system is also disclosed.