The definition of modular ontology of product knowledge was given. The idea of the product knowledge integration based on modular ontology of product knowledge was presented. An integration method based on structure-semantics-specific product knowledge modular ontology was proposed. In the method, construction of the relation bridge rules, identification of the integration rules and integration implementation were established through structure, semantics and specific levels. The flow chart of the integration process was given. The achievement of the integration process was showed through examples and enterprise application.