Based on the features of distributed knowledge resources environment and taking knowledge mobility in modern design as its supporting goal, this essay focuses on research into knowledge classification systems and representation models of product design. It proposes a six-dimension design knowledge classification system and a cubic knowledge representation model operating in an environment of distributed resources. The six dimensions of design knowledge classification are discipline, product, resource unit, application process, expression pattern, and design domain respectively. Accordingly, the six planes of the cubic knowledge representation model are, therefore, the discipline plane, expression pattern plane, resource unit plane, product plane, application process plane, and design domain plane, clearly represent the application profiles of design knowledge in six dimensions. The six-dimension design knowledge classification system and cubic model are applied to fluid simulation implicit knowledge (expert knowledge) as a real case, and are the basis upon which an application system of fluid emulation expert knowledge service is developed. The design knowledge classification system and representation model proposed in this essay are the theoretical foundations for knowledge flow model development in modern product design and the construction of knowledge management systems for modern design platforms.