A　ball　screw　(BS)　is　a　key　functional　component　for　precision　machining　equipment,　medical　instruments,　and　intelligent　manufacturing　production　lines.　The　accuracy　reliability　of　a　ball　screw　plays　a　key　role　in　the　functioning　of　all　such　equipment.　Since　the　operating　conditions　have　time-dependent　characteristics,　the　non-constant　operating　conditions　affect　the　BS　accuracy　decay.　In　this　paper,　the　numerical　modeling　and　analysis　of　the　BS　accuracy　decay　are　considered　under　non-constant　operating　conditions.　The　BS　accuracy　reliability　under　single　and　multiple　non-constant　operating　conditions　is　investigated.　The　experimental　findings　on　accuracy　decay　under　non-constant　operating　conditions　validate　the　proposed　analysis.　The　average　relative　error　of　accuracy　decay　between　the　theoretical　and　the　experimental　findings　is　estimated　at　10%　(in　the　range　of　10.15%-12.66%).　The　error　is　evaluated　under　the　three　different　amplitudes　operating　conditions,　including　AL　(non-constant　axial　load),　FR　(non-constant　feed　rate),　and　AL　plus　FR.　The　results　show　that　the　accuracy　decay　prediction　model　successfully　predicts　the　BS　accuracy　reliability　under　non-constant　operating　conditions.