Abstract: The insulation failure of turn-to-turn insulation of dry-type air-core reactors under impulse voltages frequently occurs, but the mechanism of dielectric breakdown is still unclear. In this paper, samples of two-layered polyester film aged at different temperatures were prepared, and the variation of normalized trap depth of samples was analyzed by the isothermal relaxation current (IRC) method. Moreover, the dielectric properties and impulse breakdown characteristics of samples were studied. By establishing a frequency-dependent model of absorbed energy during the forced vibration process of trapped charges in solid dielectrics, the impulse breakdown characteristics were numerically analyzed and calculated. The experimental results show that, with the increase in aging time, the normalized trap depth of the dielectric increases first and then decreases, the dielectric loss tangent tanδ gradually decreases in the high frequency region, and the impulse breakdown field strength gradually increases. Numerical calculation shows that, during thermal aging, the increase in the normalized trap depth and the decrease in the dielectric loss tangent tanδ are beneficial to the increase of the impulse breakdown field strength.