The paper intends to analyze the elastic performance and application of defamiliarization aesthetic theory in flat style illustration. With the qualitative research method, the internal logic of illustration design is supplemented by defamiliarization theory, and it is proposed to explore the elastic performance path of flat style illustration design from three dimensions of imagery, composition and color block. By demonstrating the design process of the flat style illustration of "Space Lighthouse", the research on the elastic design of flat style illustration was verified to confirm the validity of the elastic expression path of illustration design and to provide a new thought for future illustration design. The elastic expression path of illustration design, that is, the changeable space reserved for the form and concept of picture expression, can be flexibly adapted to the changing needs by reassembling the picture according to the changing laws through addition, subtraction and deformation. The elastic law of change provides a new insight into illustration design, and new approach to the formal expression of creative design while enhancing the depth and connotation of illustration design.