Bearing　stiffness　exhibits　non-linear　characteristic　when　the　bearing　is　subjected　to　a　certain　external　load,　which　will　inevitably　cause　changes　in　the　spindle　stiffness,　since　bearing　stiffness　has　a　decisive　impact　on　the　spindle　stiffness.　In　this　paper,　the　axial　stiffness　softening　and　hardening　characteristics　of　machine　tool　spindle　were　studied.　A　novel　spindle　axial　load　automatic　applying　device　and　the　spindle　stiffness　test　bench　were　proposed.　Meantime,　an　axial　stiffness　model　for　the　experimental　spindle　was　established　based　on　the　bearing　load-displacement　model　and　mechanical　analysis　of　spindle.　Then,　the　axial　stiffness　of　fixed　position　preload　spindle　under　different　preload　was　measured　experimentally　and　the　evolution　mechanism　was　analyzed.　The　results　show　that　when　bearing　preload　reaches　a　certain　relatively　large　threshold,　a　sag　shape　occurs　in　the　axial　stiffness　curve,　indicating　the　stiffness　hardening　characteristic　of　the　spindle.　On　the　other　hand,　for　a　small　preload,　no　sag　shape　occurs　in　spindle　stiffness　curve,　indicating　the　stiffness　softening　characteristic　of　the　spindle.　This　phenomenon　is　of　great　significance　for　the　acquisition　of　the　excellent　spindle　stiffness　properties.