Abstract: Ultra high speed permanent magnet motor has the advantages of high energy density, high operating efficiency and portable. However, due to the low tensile strength of permanent magnet materials, it is difficult to bear the huge centrifugal force of the rotor under the ultra-high speed. In this paper, the rotor strength problem and the structural design method of the ultra-high speed micro permanent magnet motor were studied in depth. Firstly, combined with the structural characteristics of micro rotor, the analytical model of rotor stress satisfying multi boundary type was established. Secondly, the stress distribution characteristics and variation rules of rotor structures of ultra-high speed micro permanent magnet motor under different working conditions were analyzed, taking into account the mechanical interference, permanent magnet inner diameter, material characteristics and environmental temperature rise and other factors. Then, based on the regular analysis of the influence of boundary conditions and rotor structure changes on rotor strength, a 550000r/min ultra-high speed rotor structure was designed. The selection range of sleeve thickness, interference value and the characteristic evaluation of rotor stress in different speed and temperature fields were given. Finally, the prototype was developed and tested according to the design scheme. The prototype has achieved 550000r/min stable operation.