Abstract: To describe and analyze the uncertain fault relation,fuzzy fault graph was presented.Its mathematics foundation,named fuzzy Mid Set and some other relevant conceptions,were defined and analyzed.So the structure and property of fuzzy fault graph became definite.Then the method of fault diagnosis based on fuzzy fault graph was also put forward.When the method was used in a factual system analysis,the fuzzy fault diagnosis was realized preferably.The fuzzy fault graph can be used in describing the fault relationship of complicated system effectively and realizing the fuzzy fault analysis preferably.