The diaphragm coupling has used making up for the micro displacement and angular deviation between input and output terminals by elastic deformation of its geometric shape, the loading effects caused by centrifugal force, axial deformation and torque is supported and transferred effectively, it is mainly used in the field of high precision and high parameters, such as aero and gas turbine. The computational discrete model of the coupling structure of diaphragm and sun-gear shaft is established for strength analysis. Based on strength theory of rotating machinery, the static strength and fatigue strength of sun-gear shaft is analyzed with conditions of single load and multi-coupling loads, two kinds of evaluation criteria of fatigue strength are used for checking. Result shows that strength of sun-gear shaft is meet the requirements under single or multi-coupling loads within the range of working parameters. There is a nonlinear relationship between speed and stress, the great stress is appeared at the alveolar roots, its proportion of stress occupied increases alternately with coupling load. While the high-speed rotating machinery is designed future, structure stress caused by centrifugal and Coriolis force should be considered to ensure the rationality and applicability of structure design. The structure design is safe and has a large of safety margin under uniaxial and multi-axial fatigue strength criteria. The maximum shear stress is appeared at the round angle transition region of shaft shoulder, the maximum stress value of node is 137.3 MPa.