This paper aims to achieve personalization and redesign of existing products by digging out the relationship between personalized demand and product functions. Firstly, the function of the target product was analyzed, and the function tree model and function affair-element model of the product are established. Secondly, obtain the personalized demand, refine the core demand function, and establish a demand function element. Then, with a functional body knowledge base, the two functional elements are compared, and the relationship between demand functional elements and the product functional elements is obtained. Then according to the results of matching and comparison and the needs of users, elements are extended and transformed to obtain new product functional ideas. Finally, the new structure scheme is obtained by using the method of function solving, and the design scheme is evaluated. Through the formal affair-element model, the personalized demand is integrated into the redesign of the product, which increases the operability of the design and opens up a new way of thinking for the realization of the personalized function, which will help to design the personalized products to meet the user's needs and improve the market's competitiveness. The feasibility of the design method is verified by taking the external femoral neck guide as an example.