Abstract: In this paper,the decision support in DSS is studied systematically.First,the formal representation of decision problems is presented.The decision problems are divided into two classes normative decision and descriptive decision,and their respective role is discussed.Then the two-dimensional description of support function and the unified representation of various support patterns are given based on analysis of four kinds of support patterns proposed by Keen.A new kind of support pattern,viz.,active support,is put forward.Finally,the paper introduces the function characteristics of IDDSS-MB,which is an active decision support system developed by the authors.