In order to improve the design knowledge utilization and design efficiency in drilling assembly design, an intelligent design method based on ontology is proposed in this study. The method realizes the representation of the design knowledge of the drilling assembly and constructs the knowledge base of the design of the drilling assembly by constructing ontology model including borehole parameters, formation parameters, model knowledge, and drilling engineering base. In the method, the semantic mapping relations among ontologies are built. Thus an intelligent drilling assembly design pattern was formed and the organization, push and embedding of design knowledge are realized. Combining the case-based reasoning method, the intelligent design of drilling assembly is realized.