Abstract: Bolt head simulated specimens of an aircraft connector booke in the early stage during the fatigue test,the three-dimensional nonlinear contact model for the specimens was established in order to analyzed the stress by ANSYS software,while the fracture surface of the broken bolt head was studied by SEM.The results show that the bolt fracture was due to low cycle fatigue failure in high tensile stress caused by both the bending stress cause by the thickness effect and the extrusion stress at cross section.The problem was resolved by design of compound fixture with large stiffness.