A method for predicting sulfur oxides (SOx) stored at a denitrification (DeNOx) catalyst may include calculations of the mass flow of SOx poisoned at the DeNOx catalyst, the mass flow of SOx released from the DeNOx catalyst, and the SOx amount poisoned at the DeNOx catalyst by integrating the value obtained by subtracting the released mass flow of SOx from the poisoned mass flow of SOx. An exhaust system using the method may comprise an engine having a first injector, an exhaust pipe, a second injector mounted at the exhaust pipe and injecting a reducing agent, a DeNOx catalyst mounted at the exhaust pipe and reducing SOx or nitrogen oxides (NOx) or both contained in the exhaust gas by using the reducing agent, and a control portion electrically connected to the system and performing the calculations and controls.