Based on studying design behavior and design thinking, a design behavior model which dealt with person, QCTS (quality, cost, time and service), process and environment in product design was set up. The key factors and connotation of product design were discussed based on the model. An expanded design environments model and a systematic design process model were set up. The results show that person, product, process and environment are key factors of product design; in design, the person is body of thinking, QCTS is goal, the process is behavior formula, and the environment is restraint; the product design is a process driven by customers’ requirements in which the knowledge is used and is materialized based on brain, and abstract or detail concepts are changed to a physical assembly or a product step by step.