Various　deterministic　and　uncertain　factors　are　widely　present　in　mechanical　equipment　and　their　working　conditions.　Attenuation　of　the　contact　stiffness　of　bolted　joints　caused　by　bolt　loosening　will　directly　affect　the　dynamic　characteristics　of　the　joint　surface　and　the　working　state　of　the　equipment.　This　paper　proposes　a　contact　stiffness　attenuation　model　that　considers　time-varying　uncertainty　of　the　bolt　pre-tightening　force　as　a　theoretical　basis　for　designing　the　initial　bolt　pre-tightening　force　and　secondary　tightening　strategies.　First,　the　microscopic　contact　mechanism　of　the　bolted　joint　surface　was　revealed,　and　then　a　joint　contact　load　and　contact　stiffness　model　based　on　Hertz　contact　theory　and　fractal　theory　was　established.　A　nonlinear　mechanical　model　of　the　bolted　contact　surface　was　obtained.　Considering　the　decrease　in　bolt　pre-tightening　force　during　service,　a　contact　stiffness　attenuation　law　for　bolted　joints　is　explored,　and　a　model　of　the　connection　reliability　of　group　bolt　joints　with　time-varying　uncertainty　is　established.　The　bolt　pre-tightening　force　of　a　railway　locomotive　was　taken　as　an　example　and　the　connection　reliability　of　group　bolt　structures　was　predicted,　and　it　is　defined　that　the　bolted　structure　fails　when　the　contact　stiffness　drops　to　80%　of　the　initial　value,　and　the　reliability　of　the　group　bolts　will　drop　sharply　on　the　15th　day　after　service,　and　will　almost　completely　fail　on　the　22nd　day.　The　proposed　theory　improves　the　accuracy　of　contact　modeling　and　provides　a　new　direction　for　bolt　tightening　strategies.