As critical components for high-voltage and high-current connections in power grids, ultra-high voltage (UHV) bushings rely heavily on the reliability of their epoxy resin-impregnated paper composite insulation cores. During stages such as manufacturing, installation, and operation, these cores may undergo rapid temperature variations, leading to a significant increase in internal thermomechanical stress. When this stress exceeds the material’s tolerance limit, it can cause mechanical cracking or even insulation failure. To address the inadequacy of conventional electrical performance tests at room temperature in evaluating cracking risks under rapid thermal cycling, this paper proposes an integrated assessment method combining special rapid temperature rise tests, electrical tests, and ultrasonic non-destructive inspection. Based on the extreme temperature differences and temperature rise rates that may occur throughout the bushing’s lifecycle—including manufacturing, transportation, storage, installation, testing, and operation—a series of rapid thermal cycling tests simulating various harsh conditions are designed. By comparing electrical performance measurements and ultrasonic non-destructive testing results before and after the tests, the tolerance of large-scale epoxy resin cores under rapid temperature rise condition is effectively evaluated. The research demonstrates that the proposed method offers a practical and effective approach for assessing the electrical and mechanical reliability of epoxy resin-impregnated paper cores in UHV bushings under conditions such as rapid ambient temperature rise or abrupt load current increase.