Abstract: Fatigue tests for determining the preload relaxation of composite bolted joints under transverse cyclic loading and exploring the effects of fatigue damage and fretting wear on the preload decay were carried out. The Archard wear model and the ALE (Arbitrary Lagrangian-Eulerian) adaptive meshing technology were used to calculate the fretting wear between contact surfaces and were implemented in the finite element code ABAQUS with UMESHMOTION subroutine; a progressive fatigue damage model proposed by Shokrieh and Lessard was employed to perform the failure analysis, and material property degradation was programmed by using UMAT subroutine in ABAQUS. Based on this analysis, the mechanism of the preload changing under cyclic loads attributes to the coupling action of the fatigue damage in bolt connection, the bolt-hole elongation and the contact wear. Compared with the experimental results, the rationality and validity of the proposed method were verified.