For predesigned high speed permanent magnet machines (HSPMMs), the accurate evaluation of rotor strength is extremely important to ensure the reliability of the rotor at high speeds. This paper mainly addresses a comprehensive study on the rotor stress of one predesigned HSPMM with predetermined dimensions, by considering the effect of the assembly gaps between the segmented PMs and between the PMs and the pole filler, and temperature gradient in the rotor. First, the influence of the assembly gaps for different rotor structures, different PM segments, different pole fillers, different material properties on the rotor stress are summarized by Ansys Workbench. Then, the full investigation on the rotor stress distribution is performed under the influence of the rotor temperature gradient, which is obtained by Ansys-Cfx. And then, by considering the non-isothermal distribution of rotor temperature, the 3D temperature-stress coupling analysis is performed to obtain the optimal sleeve thickness. After fabricating the prototype, continuous operation test is carried out, which validates the effectiveness of aforementioned theoretical analysis.