Using averaged discretization method (ADM) or outer discrete method (ODM) to discrete probability box can cause its compactness or inaccurate. The feasibility of combining ADM and ODM is demonstrated by analyzing the relationship between the two methods. The unified processing framework is obtained through the pre-processing of the two methods before discretization. An improved new method is proposed, in which the cumulative probability distribution of the extreme points is properly treated. The effectiveness of the improved method is demonstrated through simulation experiments. The results show that the improved method can effectively deal with discretization shortcomings, and the advantages of the ADM method and the ODM method are preserved.