Objective To study distributions of stiffness and stress of each component in internal fixation system for long bone fractures composed of fractured bones (including callus) and bone plates,and propose a method for determining bending stiffness of fracture internal fixation implants. Methods Based on the linear bending theory and composite beam theory, the mechanical model of femoral shaft internal fracture fixing system composed of fractured bones and bone plate was constructed and verified with the finite element model based on artificial femur CT data. The relationship between stress and stiffness of the internal fixation system for long bone fractures was established. Results The three stages (slow-rapid-steady growth) for variation of fractured bone bending stiffness with callus elastic modulus obtained by calculation and analysis were consistent with the three stages of actual bone healing process, which proved accuracy of the theoretical calculation method. Changing patterns for the neutral axis position of internal fracture fixing system composed of fractured bones and bone plate were further obtained, namely, the neutral axis position was close to (away from) the central axis of bone plate when stiffness of bone plate increased (decreased). According to the stress required for fracture healing (0.72-0.80 MPa), the reasonable range for bending stiffness of bone plate was 0.99-4.20 kN·mm2. Conclusions When elastic modulus of the callus was 0.037.00 GPa, bending stiffness of the fractured bones increased rapidly, which was at the stage of bone bridge formation. When elastic modulus of the callus was above 7 GPa, the callus tended to mature. Based on the femur model in this study, when thickness of bone plate was 3.6-6.0 mm and elastic modulus of fractured bones was 3.013.5 GPa, the entire fractured bone section could be effectively stimulated. In this study, when bending stiffness of bone plate was 0.99-4.20 kN·mm2 during the generation stage of bone bridge, it was conducive to bone healing.