Objective To study the biomechanical reasons of internal fixation failure. Methods On the bases of FEM stress analysis of internal fixation of femur fractue with common compressing steel plate, 34 internal implant failures were reported. Results Besides the manipulation of operation and the limitation of internal implant itself, many factors such as the position and the length of steel plate, screws' not being set completely, improper selection of screw, gaps in bone cross - section, and defects of cortical bone all have important influence on the stability and safety of steel plate. Conclusion Biomechanical influence should be emphasized in the internal fixation of bone fracture, stress concentration should consider properly, so that the rigid and effective fixation function of internal implant could be ensured.