Abstract: The cause and mechanism of abnormal heating of composite insulators are still unclear. In this paper, the temperature distribution and transient process of core sheath interface defect heating composite insulator are studied. The heat source is set at the interface, and the fiber Bragg grating is arranged at the interface, surface and test environment respectively. A 10 kV composite insulator with interface heat source and capable of measuring interface temperature, surface temperature and environmental temperature is fabricated, and the fiber Bragg grating temperature calibration experiment is carried out. Temperature detection tests under different interface heat and simulation comparative study are performed. The results show that the transient processes of interface temperature and surface temperature with the duration of interface heat source accord with the exponential function model, and the time constants are about 40 s and 115 s, respectively. Moreover, the starting time and stable time of the transient process of interface temperature and surface temperature are proposed. It is found that, under the interface heat, it takes 15~50 min and 15~35 min to stabilize the interface temperature and surface temperature, respectively. The greater the interface heat is, the longer the time to stabilize will be. The test and simulation show that the transient changes of interface temperature and surface temperature are linear with the interface heat. The simulation shows that the spatial temperature distribution range of insulator under the test interface heat in this paper is 9.5 mm in the direction of cross-section symmetry axis and 7 mm away from the heat source in the circumferential direction of interface. The heat conduction is 12 mm away from the heat source in the axial direction of the interface. This paper provides a research basis and technical means for the detection, mechanism and influence of abnormal heating temperature distribution and transient process of composite insulator.