High　speed　motor　has　the　advantages　of　small　size　and　large　power　and　is　widely　used　in　the　industrial　field.　To　design　a　motor　for　an　air　bearing　performance　test　bed,　the　motor＇s　speed　is　required　to　be　50000　r/min,　and　the　power　is　20　kW.　Bearing,　as　an　important　part　of　rotor　system,　which　working　performance　and　life　will　affect　the　reliability　of　the　whole　motor.　Therefore,　bearing　must　be　selected　reasonably　according　to　the　working　load,　working　speed,　etc.　Which　is　a　necessary　condition　to　ensure　the　normal　operation　of　the　motor,　so　as　to　effectively　improve　the　motor　life.　At　the　same　time,　the　high　running　speed　makes　the　motor　more　and　more　demanding　on　the　rotor　dynamic　characteristics.　In　order　to　avoid　the　resonance　caused　by　the　coincidence　of　the　rated　speed　and　the　natural　frequency　of　the　rotor,　the　dynamic　characteristics　of　the　rotor　were　analyzed　by　Ansys,　and　the　modal　shape　of　the　rotor　under　the　corresponding　working　state　was　obtained.　[1]　According　to　the　analysis　of　Campbell　diagram,　the　resonance　frequency　of　the　rotor　is　much　higher　than　50000　r/min.　In　order　to　reduce　the　design　margin,　the　first　six　natural　frequencies　are　calculated　under　different　bearing　steel　stiffness,　providing　a　reference　for　bearing　stiffness　selection.　©　2020　IEEE.