Composite insulators for transmission lines are one of the critical components in power systems, Brittle fracture of the core rod is one of the most severe defects in composite insulators during operation. A power frequency withstand voltage test was conducted on short samples of 1000kV composite insulators with brittle degradation, and the heating characteristics after core rod degradation were analyzed by observing infrared thermography. By analyzing the physical and chemical properties of the abnormally heated areas of the composite insulators and conducting finite element simulation calculations under different brittle conditions, the causes of abnormal heating in degraded composite insulators were identified. Test and simulation results indicate that severely degraded composite insulators exhibit significant abnormal heating phenomena, characterized by a wide heating range, high peak temperature rise, and heating not limited to the high-voltage end. The insulation resistance of the core rod significantly decreases after brittle fracture, and the dielectric loss angle significantly increases, leading to an increase in leakage current. Core rods with varying degrees of brittleness exhibit different levels of heating, as the conductivity and dielectric loss angle increase, the heating phenomenon intensifies.