Abstract: Based on the design and manufacture of the device of bolt loading, the fatigue tests of self-loosening of bolted joints under axial vibration were carried out, and the evolution curves of the clamping force of the bolts were obtained. And then the damage of the contact thread surface of bolts was analyzed, and the self-loosening mechanisms of bolt joint were discussed in detail. The results show that the clamping force of bolt joints decreased obviously, however, the breakaway torque had no obvious change compared to the pre-tightening torque. The self-loosening process of a bolted joint can be divided into two stages: firstly, the clamping force decreased greatly due to the cyclic plastic deformation of the materials, secondly, the clamping force reduced slowly caused by the fretting wear between contact threads. The wear mechanisms of the contact threads were mainly adhesive wear, abrasive wear and delamination.