The　bolted　joint　is　widely　used　in　heavy-duty　CNC　machine　tools,　which　has　huge　influence　on　working　precision　and　overall　stiffness　of　CNC　machine.　The　process　parameters　of　group　bolt　assembly　directly　affect　the　stiffness　of　the　connected　parts.　The　dynamic　model　of　bolted　joints　is　established　based　on　the　fractal　theory,　and　the　overall　stiffness　of　joint　surface　is　calculated.　In　order　to　improve　the　total　stiffness　of　bolted　assembly,　an　improved　particle　swarm　optimization　algorithm　with　combination　of　time-varying　weights　and　contraction　factor　is　proposed.　The　input　parameters　are　preloading　of　bolts,　fractal　dimension,　roughness,　and　object　thickness.　The　main　goal　is　to　maximize　the　global　rigidity.　The　optimization　results　show　that　improved　algorithm　has　better　convergence,　faster　calculation　speed,　preferable　results,　and　higher　optimization　performance　than　standard　particle　swarm　optimization　algorithm.　Moreover,　the　global　rigidity　optimization　is　achieved.