Abstract: Fracture is a common orthopedic disease, and internal fixation implants are often used in the treatment of fractures. In the process of tissue healing or repair at the fracture, the structure mechanical properties of ideal fracture internal fixation implants need to meet different biomechanical requirements of different fracture healing stages. A microstructure topology optimization design method for regulating and designing the time-changeable stiffness characteristics of the biodegradable composite microstructure is proposed to meet the special time-changeable stiffness characteristics requirements of the ideal fracture internal fixation implant. Two kinds of degradable materials with different degradation rates and stiffness are used, and the relative density is used as the design variable to describe the distribution of different materials. Taking the maximum sum of the stiffness of the intermediate structure in specific degradation steps as the optimization objective, the topology optimization design of microstructure configuration of the composite is carried out to make it have a specific time-changeable characteristic in line with the law of fracture healing. Using the uniform corrosion method, the degradation process of the composite structure is described by material residual rate which is time-dependent, and the finite element model considering material degradation in the time dimension is established. The continuous degradation update formula is proposed by integrating the Heaviside function and Kreisselmeier-Steinhauser function. The mechanical properties of the intermediate structure at different degradation time steps are calculated by homogenization method, and the sensitivity of the optimization objective to the design variables is computed. Compare with the structure using only a single material and the topology optimization structure without time-changeable characteristic regulation, the effectiveness of the proposed topology optimization design method of composite material microstructure considering time-changeable stiffness characteristics is verified. And the effects of different parameters on the optimized configuration of unit cell and time-changeable stiffness characteristics are also studied.