Abstract: The knowledge representation and knowledge-base organization, the flow of matching welding procedure specification and realizing technology for reasoning in the developed expert system for constituting arc welding process based on JB4708-2000, which is a standard of welding procedure qualification for steel pressure vessels, are introduced. The software system is developed by Visual C++ completely, in which the CExper C++ class base being developed by University of Edinburgh is used to realize the base reasoning operations such as matching, recursion and backdating. The logic reasoning and the user interface are combined together in the software, and the software system has strongly practicability.