The　heavy　load　mechanical　spindle　is　an　important　functional　component　in　a　5-axis　computer　numerical　control　(CNC)　machine　tool,　which　is　used　to　process　large　and　complex　free-form　surfaces.　It　is　necessary　to　obtain　the　natural　frequency　and　analyze　the　spindle　stability　for　improving　the　machining　precision.　In　this　paper,　Timoshenko　beam　theory　is　introduced　to　model　the　mechanical　spindle　shaft,　where　the　centrifugal　force　and　gyroscopic　effects　are　considered.　Stability　of　the　heavy　load　mechanical　spindle　shaft　is　analyzed,　and　the　buckling　load　of　the　spindle　shaft　is　obtained　under　different　rotational　speeds.　The　natural　frequency　of　spindle　is　investigated　in　a　freedom　and　restraint　state,　respectively.　Comparing　the　proposed　method　with　the　simplified　hollow　cylinder　and　shaft　prototype　in　the　freedom　state,　the　results　show　that　they　are　highly　correlated　with　experimental　results.　For　the　restraint　state,　the　axial　load,　rotational　speed,　gyroscopic　effect,　and　centrifugal　force　are　discussed,　and　all　of　these　parameters　affect　the　natural　frequency.　The　proposed　modeling　approach　can　be　used　for　spindle　design　and　optimization　in　a　given　machining　process　and　can　be　easily　extended　to　other　spindle　design.