Abstract: In fault diagnosis with D-S for a complex system,such as practical missiles and weapon systems,sometimes the result provided by perts in this field is only some range of the impersonal value or some fuzzy value of basic belief assignment because of the increasing factors that affect basic belief assignment.This paper puts forward a kind of fault diagnosis method with D-S for a complex system.First,a fault decision-making method with IBBA is given according to the diagnosis knowledge and experience of experts in this field that is combined with the decision-making method of D-S evidence theory.Then according to the purpose of fault diagnosis,a diagnosis method with D-S based on IBBA is introduced with the use of optimization theory.The principle of which is elaborated and the object function and constrain function are also constructed.Finally,some examples prove the rationality of the proposed fault decision-making method and diagnosis method with D-S.