The static stress and dynamic stress field of gear shaft considering centrifugal stress are simulated by explicit dynamic FEM and implicit static FEM for the crack problems occurred in the impact load testing for accessory gearbox of a turbojet. The simulation results show a good correspondence with the measurement results. According to the results of the analysis, the dynamical parameter model of gear shaft failure parts is built. When the weight of the gear axis is treated as optimization objective, using APDL the failure parts is optimized. The results indicate that the optimized structure is obtained, whose weight get to minimum with constrains of strength and stiffness. Under the impact load, the dynamic stress substantially increased than the static stress. In order to ensure safety margin of structural strength, the dynamic stress caused by impact load must be considered in the design of gear shaft.