An after treatment method for a lean-burn engine is disclosed.The after treatment method is configured to control an after treatment system sequentially equipped with an ammonia production catalyst module, a selective catalytic reduction catalyst, and a CO clean-up catalyst on an exhaust pipe through which an exhaust gas flows. In the after treatment method, the engine is operated sequentially at a stoichiometric air/fuel ratio (AFR) and a lean AFR prior to entering a rich AFR.