The invention discloses a fault diagnosis expert system based on a decision tree for an industrial Ethernet network. Firstly, an expert system comprising a knowledge base, an inference engine, a knowledge base management module, a knowledge acquisition module, an explanation facility and a control center is established; secondly, the knowledge base is utilized to contact the inference engine and the control center to obtain data required by the modules for storing a diagnosis rule, various pieces of data of the system and an intermediate result generated during the system diagnosis period; thirdly, comparison, commonly called as matching, is carried out between a condition part of a rule base and a content of a global data base through the inference engine, if matching is successful, a conclusion part is displayed, the global data base is modified according to an action part of an enable rule, the changed global data base can trigger a new rule, so that problem solving proceeds to the next state, and so forth, one problem is finally solved; and lastly, post processing is carried out through the inference engine, a new knowledge base is updated by the control center, so that the expert system is gradually improved.