Function-behavior-structure (FBS) method models the functional structure and principle scheme in conceptual design by connecting three variables: product function, behavior, and structure. The traditional FBS method cannot deal effectively with the cooperation problem between man and machine in the function achievement process. This paper proposes an FBS-based interactive mechanical product design method by extending the behavior layer. First, define and transform operational behavior from the function to the behavior layer. Second, the genetic engineering method is used in expected operational behavior design. In this part, the operational behavior as genetic information is constructed to operational behavior genes model by Extenics theory. Based on a homology product sample library, screen the target gene through the GloVe model and similarity algorithm. The original operational behavior is then modified to an expected one by Extenics transformation. Finally, the product structure is obtained through an analogy transformation process. This way, the operational behavior problem in the interactive mechanical product can be solved by this FBS design process. The feasibility and effectiveness of the proposed process are verified in the development of an apple picking device.