The fuzzy logic and TS fuzzy model are introduced into the fault tree analysis， and TS fuzzy door is used to describe the relations of events， to solve the uncertainty of events relations. TS fuzzy door is applied in fault tree， and this method is proved to be practical. Both causality diagram and fault tree can be applied in fault diagnosis， and the causality relation can be expressed by diagram. There is a strong resemblance between fault tree and causality diagram， and under certain conditions， fault tree and causality diagram can be transformed into each other. ［1］ Due to some limitation of this paper， causality diagram in this paper is all certain relation. From the literature［2］， causality diagram transformed from fault tress with no coupling is causality tree， so on the basis of the application of TS fuzzy door［3］ in fault tree， TS fuzzy door is introduced into causality diagram.