Abstract: This paper first analyzes the SDG method with a view of knowledge description and management,then presents an improved way according to the characteristics of the liquid rocket engine systems.That is,by constructing the SDG containing only the measured variables,the fault modes are diagnosed in advance,and then the modes that can not be distinguished are diagnosed by adding adequate quantitative information.In addition,multi fault mode diagnosis method is proposed in this paper.The test result shows that the method is valid.