An exhaust purification system for an internal combustion engine, and a desulfurization method for the same, recovers purification performance of a denitrification catalyst by removing sulfur poisoned at the denitrification catalyst in an exhaust system. The exhaust purification system may include an exhaust pipe through which an exhaust gas flows, the exhaust gas being generated at the internal combustion engine having a first injector injecting a fuel to a combustion chamber, a second injector mounted at the exhaust pipe and additionally injecting the fuel, a particulate filter mounted at the exhaust pipe downstream of the second injector and filtering particulate matters (PM) in the exhaust gas, a denitrification catalyst mounted at the exhaust pipe downstream of the particulate filter and reducing nitrogen oxide contained in the exhaust gas, and a control portion performing desulfurization of the denitrification catalyst when a desulfurization entering condition and a desulfurization condition are satisfied during regenerating the particulate filter.