An operation aiding expert system is developed for preventing an unexpected shutdown caused by operation errors or misjudgements in boiler plants. Based on symbolic model about the plant structure and the behavior characteristics, the change of process states is simulated. And the method for solving context-dependent problem is suggested. With generalized knowledges for the recognition of operational situations and hazardous conditions, a strategy is suggested for anticipating accidents caused by operation errors or misjudgements in advance. By the results of the application of this system to the real plant, it is shown that the methodology suggested in this study is useful.