With the increasing of power grid capacity and voltage level, the serious accidents caused by a large power transformer short-circuit are increased, and this situation has a direct threat to the safe operation of the grid. Through analysis of the large power transformer leakage flux and short-circuit electromagnetic forces, the radial mechanical instability of transformer inner winding is pointed out, then a radial electromagnetic force calculating and an assessment of ability to withstand short-circuit on inner windings are researched. Moreover, the critical destabilization strength of winding pies and assessment process of the ability to withstand short-circuit for large power transformers are given. At last, the improvement measures to increasing the ability to withstand short-circuit for transformers are proposed based on case studies and four kinds of comparative analysis.