Field infrared inspection has become one of the most effective approaches to screen out the faulted insulators during operation, but the influence factors of the abnormal heating characteristics of the in-service composite insulators still remain unclear. In this paper, 11 composite insulators with abnormal heating behaviors during operation were acquired after ceasing operation, and were re-energized in the high voltage (HV) test hall to acquire the temperature rise (TR) characteristics in different conditions. The result indicates that the ambient condition influences the number of TR segment, but has no influence on the TR location of each segment. More importantly, the TR characteristics of the samples were acquired after removal of silicone rubber (SR) housing to study the abnormal heat source. Similarly, for some samples, the numbers of TR segment were reduced compared to the test results with SR housing, but the TR location is stable. Remarkable TR was recorded for all samples after removal of SR housing, and large leakage current was detected at the HV end of selected samples. These evidences indicate that abnormal heating phenomenon of the tested composite insulators is likely to be ascribed to the dielectric loss of the decay-like degraded core rod.