Method of controlling an NOx purification system provided with an NOx occlusion reduction catalyst and a control unit that conducts NOx regeneration control and sulfur poisoning regeneration control. A degree of deterioration of the NOx occlusion reduction catalyst due to sulfur poisoning is estimated on the basis of an amount of an integrated value of a difference between an oxygen concentration on an outlet side of the NOx occlusion reduction catalyst and an oxygen concentration on an inlet side thereof. In so doing, the amount of sulfur poisoning in the NOx occlusion reduction catalyst is accurately estimated, excessive sulfur poisoning regeneration can be avoided, and fuel consumption can be reduced.