Abstract: Aiming at the problems of ontology application in engineering knowledge management for complex product development, a new ontology based on domain ontology and product development task, named dynamic ontology, is proposed. The paper firstly analyzes the shortages of the existing large ontology and top ontology in large-scale engineering knowledge management application, and then defines the dynamic ontology. Secondly, the preliminary method of dynamic ontology construction is described, which consists of determination of the relevant domain ontologies and their subsets, and the merging of the subsets of relevant domain ontologies. A method to evaluate the relevancy between the dynamic ontology and domain task requirements is also proposed. Rocket engine is taken as an example of complex products to elaborate the procedures of dynamic ontology construction. Four domain ontologies are taken into consideration, which are related to structure, thermodynamics, process and aerodynamics disciplines. The results of evaluation demonstrate that the dynamic ontology is more appropriate for complex product development engineering than large ontology and top ontology.