As the rated speed and power of high-speed permanent magnet synchronous machine (HSPMSM) are limited by its rotor dimension, the mechanical strength and stiffness analyses become essential. This paper researches on the influence degree of some factors on stress and critical speed, namely rotor length-diameter ratio, materials, interference fit, and bearing support stiffness. The orthogonal test method is used to achieve a sensitivity analysis of these factors. The method and conclusion may provide some references for HSPMSM optimal design.