In　the　process　of　machining,　vibration　characteristics　of　the　machine　tool　spindle　tool　system　can　be　improved　by　structure　optimization　of　machine　tool　spindle　and　appropriate　selection　of　tool　and　toolholder,　reaching　a　higher　machining　quality　and　efficiency　with　good　vibration　characteristics.　To　study　the　vibration　characteristics　of　the　machine　tool　spindle,　this　paper　proposed　a　method　based　on　combined　stepped　beam　model.　The　method　for　solving　bending　vibration　natural　frequency　and　mode　of　stepped　cantilever　beam　was　developed　with　the　use　of　Euler-Bernoulli　beam　theory.　Based　on　the　tool-toolholder　coupled　system　from　the　practical　condition,　this　paper　refined　the　stepped　beam　model　to　the　combined　stepped　beam　and　gave　the　equivalent　bending　rigidity　and　density　of　the　combined　part.　This　method　was　applied　to　calculate　the　vibration　characteristics　of　the　tool-toolholder　coupled　system　and　it　was　verified　though　the　finite　element　method　(FEM)　and　experiment.　Results　show　that　compared　with　the　Timoshenko　beam　theory,　this　method　doubles　the　computational　efficiency　whereas　calculation　error　keeps　small　in　condition　of　large　ratio　of　length　to　diameter　and　low　order　of　modal　is　considered　like　the　machine　tool　spindle　tool　system.　©2019,　Editorial　Department　of　Journal　of　Beijing　University　of　Technology.　All　right　reserved.