Abstract: An expert system of welding crack diagnosis based on knowledge base is posed according to the analysis of characteristic of weld crack.The General structure of system and object-oriented model are discussed in the paper.A data base (DB) is set up according to system demand,a case base (CB) founded through accumulating cases,rule base (KB) established after accepting expert experience and knowledge of this field,and a model base (MB) built according to the structure of knowledge.The four bases are recombined in the expert system.The diagnostic methods based on case,rule and model were described.Compared with each other,the advantage and disadvantage of these reasoning methods are obtained.The mixed reasoning based on case,rule and model is posed to overcome their shortcoming,retain fully their advantage and find the answer to the problem as possible.The definition,representation and calculation of the degree of similarity and rule representation are discussed.The function of system and way of realization are described in the paper.It is proved by application that the expert system of weld crack diagnosis based on knowledge base can achieve the same diagnostic result as the expert.