Objective To study the distribution of stiffness and stress of each component in the internal fixation system for long bone fractures composed of broken bones (including callus) and bone plates，proposed a method for determining the flexural rigidity of fracture internal fixation implants. Methods Based on the linear bending theory and the composite beam theory, the mechanical model of the femoral shaft internal fracture-fixing system composed of broken bone and bone plate is constructed and verified with a finite element model based on artificial femur CT data. Finally, the relationship between stress and stiffness of the internal fixation system for long bone fractures is established. Results The three stages (slow growth-rapid growth-steady growth) of the bending stiffness of the broken bone obtained by calculation and analysis with the elastic modulus of the callus are consistent with the three stages of the actual bone healing process, which prove the accuracy of theoretical calculation methods. Further obtain the change rule of the neutral axis position of the fracture internal fracture-fixing system composed of broken bone and bone plate, increasing (decreasing) the rigidity of the bone plate can make the position of the neutral axis approach (away) to the central axis of the bone plate. According to the stress required for fracture healing (0.72~0.8MPa), the reasonable range of flexural rigidity of the bone plate is 990~4200 N·mm2. Conclusions 1) When the elastic modulus of the callus is 0.03~7GPa, the flexural rigidity of the broken bone rises rapidly, which is the stage of bone bridge formation. When the elastic modulus of the callus is above 7GPa, the callus tends to mature. 2) Based on the femur model in this article, when the thickness of the bone plate is about 3.6-6mm and the elastic modulus of the broken bone is about 3~13.5GPa, the entire fractured bone section can be effectively stimulated. 3) The research object of this article is in the generation stage of the bone bridge, when the bending stiffness of the bone plate is 990~4200 N·mm2, it is conducive to bone healing.