Epoxy resin is widely used in high-temperature, high-electric-field power equipment owing to its stable physicochemical properties and excellent insulation performance. During prolonged operation, it undergoes thermal aging, making lifetime extension essential. The effects of thermal aging on insulation by comparing samples cured with anhydride curing agents under both air and non-air conditions are investigated. The Fourier transform infrared spectroscopy, current integrated charge, and pulsed electro-acoustic methods are used to analyze physicochemical characteristics and electrical performance. Control groups compared thermal aging in air versus an oil bath to examine atmospheric influences. Results show that air-aged samples turned yellow, while the oil bath-aged samples remained colorless. At temperatures above T_g (135 ℃), air-aged samples showed more positive hetero charge accumulation. Non-air-aged samples also accumulated space charge, but to a lesser extent, and showed better insulation breakdown resistance. These findings indicate that thermal aging in air may degrade insulation performance owing to oxidative effects.