A system includes a vehicle operation detector, an exhaust brake operation unit, a diesel oxidation catalyst, a diesel particulate filter, an injector, and a controller for controlling the operation of the injector and the exhaust brake operation unit. A post injection control method during exhaust brake operation for the system includes: determining whether a predetermined regeneration condition of the diesel particulate filter according to the output signals of the vehicle operation detector is satisfied; performing, by the controller, regeneration by controlling the operation of the injector if the regeneration condition is satisfied; determining, by the controller, whether an exhaust brake operation condition is satisfied during regeneration based on the output signals of the vehicle operation detector; and controlling, by the controller, the operation of the injector to perform post-injection with a predetermined amount of fuel when the exhaust brake operation condition is met during the regeneration.