The main cause of transformer fault is insulation aging and damage caused by internal oil-paper insulation system under thermal and mechanical stress.The effect of thermo-mechanical combined stress on oil paper insulation is studied.Oil-impregnated paperboard is subjected to thermal aging at different times and different forms of mechanical stress are applied, and the strain generated by the paperboard under stress is measured.The effects of different loading conditions on the characteristics of withstand time, phase resolved partial discharge(PRPD) spectrum, average discharge capacity and other partial discharge characteristics were analyzed.The longer the stress loading time, the higher the stress amplitude or the lower the stress frequency, the more severe the damage of the oil-impregnated paperboard under the same mechanical stress, and the worse the mechanical insulation and mechanical properties of the paperboard.Under the action of thermo-mechanical accumulation, the effect of different mechanical stress is similar to that of unaged, but the morphology of PRPD spectra after thermal aging is mostly "rabbit-like".With the increase of thermal aging time, the insulation life of paperboard first increased, then decreased.The results of this study will provide a reference for establishing an aging model of oil-impregnated paperboard under thermo-mechanical combined stress.