Precision　double-nut　ball　screws　are　key　functional　parts　in　machine　tools.　The　positioning　accuracy　of　a　ball　screw　can　directly　affect　the　machining　accuracy　of　a　machine　tools.　During　actual　use,　the　preload　degradation　and　the　large　axial　clearance　of　the　double-nut　ball　screw　are　caused　by　the　contact　wear,　which　directly　affects　the　transmission　positioning　accuracy.　The　contact　wear　is　focused　on　the　ball　and　the　rough　raceway　surface　of　the　ball　screw.　Therefore,　a　novel　method　is　proposed　in　this　paper　to　characterize　the　rough　spherical　morphology　of　raceways　for　ball　screws　based　on　fractal　theory,　and　the　fractal　parameters　of　the　screw　and　nut　raceways　are　identified.　A　fractal　contact　model　of　the　asperity　on　the　raceway　surface　is　established.　By　introducing　contact　parameters　to　modify　the　contact　area,　an　area　distribution　function　suitable　for　the　coordinated　contact　is　obtained.　A　mathematical　model　for　calculating　the　normal　contact　load　of　the　ball　screw　in　three　states　is　established　by　considering　the　friction　factor.　A　precision　loss　model　of　the　ball　screw　is　established　based　on　the　full　ball　load　distribution　model　and　the　multiscale　contact　load　model,　and　the　proposed　model　is　verified　by　experiments.　The　surface　contact　coefficient　of　the　inner　and　outer　raceways　is　discussed.　The　influence　of　raceway　fractal　parameters　on　the　wear　rate　is　analyzed,　and　coupling　research　on　precision　loss　and　preload　degradation　for　the　ball　screw　is　carried　out.　The　initial　wear　rate　under　different　axial　load　is　discussed,　and　the　change　trend　of　wear　rate　with　time　is　analyzed.　The　precision　loss　model　of　the　preload　double-nut　ball　screw　based　on　fractal　theory　provides　a　new　theoretical　basis　for　accurately　predicting　the　precision　degradation　and　provides　method　support　for　establishing　the　wear　life　prediction　of　a　ball　screw.