A desulfurization method of a nitrogen oxide absorption catalyst when diesel is used may include determining how many times a regeneration of a diesel particulate filter (DPF) is completed, ending a DPF regeneration, if the number of times of the DPF regeneration reaches a predetermined value and entering into a desulfurization mode to desulfurize the DPF, ending the desulfurization mode after the desulfurization mode is performed for a predetermined time, and calculating a particulate matters (PM) amount that is trapped in the DPF after the desulfurization, compensating the trapped PM amount, and determining a time of the DPF regeneration. A desulfurization timing is determined based on the number of times that the DPF is regenerated to be able to simplify the desulfurization logic and also reduce the memory of ECU, when the LNT catalyst is poisoned by a small amount of sulfur included in exhaust gas.