Dissimilar safety assessment systems use different conception and terminologies to describing the same object domain by respective system engineers, thus it is not easy to extract the knowledge from one system and apply it in another. An ontology-based method for construction of safety assessment system is introduced for power plants in this paper. The domain ontology knowledge base of different evaluated objectives is constructed according to Skeletal methodology, and providing an ontology model of fault characteristic for equipments that can be shared and reused in power plants. The proposed method can be used to the existing safety assessment system, reducing the fuzziness of primary information and enhancing the accuracy and credibility of safety assessment system.