It studies the evaluation method of products design knowledge based on priority degree theory. Directed at knowledge characteristics，on the basis of the comparison among relevant studies， it makes clear the basis process of the evaluation demands， purpose and procedures.Combined with analytic hierarchy process， it also establishes a comprehensive product knowledge evaluation index system and acquires the evaluation indication，on the basis of which，this paper establishes correlation function and acquires the optimal knowledge object through priority calculating. Taking the systematic knowledge of a certain type of liquid tank-type semi-trailer as example， it acquires the systematic knowledge which best meets the demand of client′s and the design expectation， as well as tests the feasibility of the method. The evaluation method of product design knowledge based on the priority degree theory can make the evaluation and decision-making process， digital system， make the evaluation results more objectivity and dependability.