The　effect　of　preload　relaxation　caused　by　the　mechanical　vibration　on　loosening　of　bolted　joints　with　self-locking　capability　has　been　widely　observed.　This　paper　proposed　a　combination　testing　strategy　involving　tightening　and　relaxation　to　elucidate　the　loss　mechanism　of　bolt　preload.　A　logarithmic　relationship　between　torque　coefficient　and　tightening　torque　is　reported　based　on　the　results　of　fastening　tests.　Two　types　of　specimen　were　designed　to　evaluate　the　preload　relaxation　under　axial　and　transverse　vibration.　A　time-varying　model　is　proposed　for　predicting　the　residual　preload　of　bolts.　An　orthogonal　test　involving　tightening　torque,　amplitude,　and　frequency　was　conducted.　The　results　show　that　the　relaxation　evolution　of　bolted　joints　subjected　to　vibration　is　similar　and　can　be　divided　into　two　stages.