Product design is a problem-solving activity based on knowledge. This paper is devoted to presenting a systematic knowledge representation method of principle solution based on semantic network model. For the expression of product knowledge, the semantic object, constraints and their relationships among the expression of the semantic object network model are proposed step by step. Then the principle solution representation model based on semantic network model is put forwards. The knowledge representation of a car is given as an example, which demonstrates that this method is obviously helpful for knowledge-based design system and product innovation.