The cumulative plastic deformation of the windings in transformers during short-circuit impacts is the primary cause of transformer accidents. Among these, large-capacity power transformers are more prone to significant plastic deformation during short-circuit impacts due to higher short-circuit currents. Currently, both domestically and internationally, research on cumulative plastic deformation in large-capacity transformers during reclosing operations has primarily focused on the ageing of spacers and qualitative measurements of deformation quantities, while analyses of the mechanisms underlying cumulative plastic deformation in windings and the influencing factors on structural components remain insufficient. To investigate the mechanism of cumulative plastic deformation, a computational model based on electromagnetic-structural coupling was established to simulate the cumulative plastic deformation of transformer windings under three-phase short-circuit conditions. This model was used to study the cumulative plastic deformation of large-capacity power transformers under six three-phase short-circuit impacts. The computational results show that the cumulative plastic deformation of the windings increases with the number of short-circuit impacts, while the cumulative rate gradually decreases, and the total cumulative deformation tends to saturate. Additionally, a comparison was made between the transformer under fully constrained conditions of the upper pressure plate structural components and the transformer under unconstrained conditions following the failure of the upper pressure plate. The computational results indicate that after the upper pressure plate fails, the impact of vibrations increases, enhancing the transformer"s resistance to short-circuit impacts, while the rate of cumulative plastic deformation slows down.