In order to elucidate mechanical impacts affecting the insulation strength of thermal‐ageing oil‐impregnated pressboards, we perform mechanical degradation experiments of impacting forces that are represented as the winding‐suffered stresses on thermal‐ageing oil‐paper insulation in transient short‐circuit of transformer. Water content, degree of polymerisation (DP), partial discharge inception voltage and AC breakdown field strength are tested and analysed for diverse thermal‐ageing levels. An analytical model of mechanical degradation is established by relating dielectric breakdown strength to mechanical impact. Mechanical impacts result in less water content in thermal‐aging oil‐impregnated pressboards while causing reduction and increment of DP, respectively, in mildly and severely aged oil‐impregnated pressboards. Partial discharge inception voltage and dielectric breakdown strength vary significantly with the numbers of impact cycles, showing a maximum value under low‐intensity impact and decreasing monotonously under high‐intensity impact. High‐intensity mechanical impacts will evidently exacerbate the thermal deterioration process of oil‐impregnated pressboards, which is primarily attributed to cellulose fibre fractures in pressboard cracks produced by equivalent tensile stress of mechanical impacts. It is acceptable for mildly aged oil‐impregnated pressboards to suffer low‐intensity mechanical impacts that will alleviate insulation degradation by restraining partial discharge in smaller oil‐gaps and prolonging breakdown discharge pathways along impact‐caused cracks.

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