A method of patent document classification guided by design objective was proposed. And then the support of different types of patents on product innovation was analyzed. The knowledge system for product innovation, comprised of innovation level knowledge, association knowledge, principle knowledge and patent language knowledge, was established. Based on frame structure and concept graph the representation model of patent principle knowledge was described. Furthermore, a method for patent principle knowledge acquisition of mechanical product was studied. Nature language understanding was utilized to extract patent working-structure concept graph from patent document. Domain association knowledge was employed to distill the functional effect of principle knowledge. Finally, with the conceptual innovation design of drill chuck as an example, this method was proved to utilize patent knowledge for conceptual design, and to improve product innovation capability. 