A method for predicting regeneration may include calculating total mass flow of reducing agent, calculating mass flow of the reducing agent used in nitrate reduction reaction, mass flow of the reducing agent used in NO2 reduction reaction, and mass flow of the reducing agent which is simply oxidized by using the total mass flow of the reducing agent, calculating mass flow of released NO2 and mass flow of reduced NO2 by using the mass flow of the reducing agent used in the nitrate reduction reaction and the mass flow of the reducing agent used in the NO2 reduction reaction, calculating mass flow of NO2 slipped from DeNOx catalyst, and calculating mass of NO2 and mass of NOx remaining at the DeNOx catalyst after regeneration based on the mass flow of the released NO2, the mass flow of the reduced NO2, and the mass flow of the slipped NO2.