Abstract: Excited by the magnetic forces under the action of short-circuit current, transformer coils vibrate strongly in axial direction to affect their mechanical strength. To further investigate windings' vibration characteristics during short-circuits and their influence on the axial mechanical strength, this paper firstly establishes a nonlinear axial vibration model and proposes methodologies to calculate dynamic compressive forces of coils, spacers, clamping rings, and axial bending stresses in conductors. Then, the proposed models are used to assess the dynamic axial mechanical strength of an SZ-50000/110kV power transformer, and the obtained results are compared with that from the traditional static assessment. Finally, the influences of current amplitude, clamping pressure, and natural frequencies on axial strength are discussed. The results show that the calculated compressive forces and stresses of winding disk, spacers, clamping rings and axial bending stress in conductors from the dynamic assessment are slightly higher than those of the static assessment. For the transformer used in this paper, when the second-order natural frequency gets close to the forces' frequency, the compressive force of disk increase by 10kN to 25kN, stresses of clamping rings increase by 8MPa, and axial bending stresses increase by 5MPa to 10MPa. The proposed model and methodology are valuable in improving the assessment of windings' mechanical strength.