The　simultaneous　engagement　of　planet　gears　with　the　sun　gear　and　the　ring　gear　is　the　main　reason　for　the　complicated　vibration　of　the　planetary　gearbox.　However,　the　relationship　of　planet　gear　parameters　on　the　vibration　has　not　been　effectively　revealed.　To　end　this,　the　modified　meshing　phase　model　of　gear　pairs　is　established,　and　moments　of　sudden　stiffness　changes　of　gear　pairs　are　calculated.　Furthermore,　the　problem　of　simultaneous　tooth　shifting　of　gear　pairs　and　the　number　of　sudden　stiffness　changes　in　one　meshing　period　are　also　discussed.　Then,　according　to　this　sudden　changing　number,　planetary　gearboxes　without　tooth　displacement　are　classified　into　four　types,　where　the　classification　results　are　closely　related　to　the　gear　number　and　the　teeth　number　of　the　planet　gear.　Finally,　the　dynamic　simulations　of　these　four　types　of　gearboxes　are　carried　out　in　the　finite　element　software.　Simulation　results　show　that　these　4　vibration　signals　are　different　both　in　the　time　and　frequency　domains.　©　2021　IEEE.