Taking　a　permanent　magnet　synchronous　motor　with　10　kW　and　120　000　r/min　ultra-highspeed　as　a　test　prototype,　tests　were　conducted　to　study　dynamic　behavior　of　arotor　system　with　3-pad　bidirectional　gas　foil　bearing　(GFB)　and　high-speed　motor.　The　3-pad　bidirectional　GFB＇s　load-bearing　features　were　analyzed,　natural　frequencies　of　conical　and　parallel　whirling　modes　of　the　elastically　supported　rotor　system　were　calculated　based　on　an　appropriate　simplification　ofbearing　stiffness　and　damping　coefficients.　30　000　r/min　and　60　000　r/min　speed-up　tests　and　a　90　000　r/min　free　deceleration　test　were　conducted　on　the　prototype.　Test　results　showed　that　the　take-off　speed　of　the　rotor　is　about　4　000　r/　min;　when　the　rotor　running　at　90　000　r/min,　there　is　a　strong　sub-synchronous　whirl　frequency,　it　is　completely　caused　by　the　system　itself;　the　rotor　system　with　3-pad　bidirectional　GFB　has　a　better　stability;　the　theoretical　analysis　and　test　results　here　provide　a　reference　for　designing　ultra-high　speed　permanent　magnet　motor　shafting　and　its　dynamic　analysis.　©　2021,　Editorial　Office　of　Journal　of　Vibration　and　Shock.　All　right　reserved.