In pressurized water reactors (PWRs), bolted joints, whose loosening behavior is inevitable due to flow-induced vibration, are extensively used to connect components and parts. In this work, the loosening tests combined with the finite element analysis were applied to reveal the loosening mechanisms of bolted structures in PWRs. A three-dimensional finite element model with fine mesh threads, which was validated through theoretical method and loosening tests, was established to simulate their loosening behavior. Though analyzing the changing preload and slip contact condition between threads and bolt, the effects of preload, friction coefficient, amplitude of axial excitation, temperature and cycle of vibration load on loosening behavior were investigated. The results indicated that with the increase of preload, the frictional stress increased and then lead to a significantly reduction of slip amplitude and dissipated friction energy per unit area. As a result, the damage of threads was slighter,which was agreed with loosening tests. In addition, the cumulative plastic deformation induced by axial excitation at the roots of threads decreased due to the enlargement of contact area. Consequently, the ratio of the residual axial load to preload of this bolt decreased. Remarkably, along with the increase of friction coefficient or amplitude of axial excitation, the bolt jolts were experiencing the same situation. On the other hand, with the increase of temperature, the relative motion between the contact surfaces increased due to the inconsistent thermal deformation of the bolted connection structure. Accordingly, the wear of bolt surface got worse with the increase of temperature, and the looseness of bolted connection structure increased. Interestingly, the number of cycles influenced bolt mechanical properties through accumulated plastic deformation and contact state. During loosening process, the initial stress and axial force decreased rapidly and then stabilized. Meanwhile, the displacement distribution gradually became uniform with the increase of cycles.