In energy application system, electrical energy is economic, practical, clean and easy to control and transfer. Electrical cables, especially high-voltage cables, have been extensively and increasingly applied in industries, and however their fire safety deserves more attention. Thermal behaviours of flame-retarded high-voltage Cable Sheath and Insulation were studied using SDT Q600 thermal analyser. The cable sheath pyrolysis profiles show three peaks while the insulation has only one peak. Possible reaction pathways and activation energy evolution with various conversion rates were analysed using both model-free and model-fitting methods. The zero order model and the 6th order model are responsible in two cable sheath pyrolysis stages, respectively. The one order model plays an important role in the pyrolysis of cable insulation. Evolved gas analysis by Fourier transform infrared spectroscopy (FTIR) shows that the amount of six main components produced in the order of most to least is CO2 > C-H stretching > C-H bending > C-H aliphatic bending > CH2 deformation > C-Cl stretching for the cable sheath while the relationship of C-H stretching > C-H scissor vibration > CO2 > O & C=C stretching is determined for the cable insulation.