This study aims to provide theoretical and technical support for early fire warning of high-voltage cables. Fourier-transform infrared spectroscopy (FTIR) was employed to systematically analyze the gaseous product evolution characteristics during the pyrolysis of high-voltage cable PVC sheaths in the temperature range of 90–250 °C. The results show that the absorption peaks corresponding to functional groups such as C–Cl, C–H, –OH, and = C–H significantly intensified with increasing temperature, indicating a clear temperature dependence in chain scission, dechlorination, and the release of unsaturated species during pyrolysis. Furthermore, three inorganic flame retardants—Mg(OH)2, Sb2O3, and 3ZnO·2B2O3—were introduced for comparative analysis. It was found that ZnO effectively suppressed the C–Cl absorption peak and enhanced the –OH absorption intensity, demonstrating a strong inhibitory effect on C–Cl bond cleavage and excellent synergistic flame-retardant performance. B2O3 exhibited moderate effects, while Mg(OH)2 showed relatively weaker suppression. These findings provide valuable data support for infrared spectroscopy-based identification of pyrolysis behaviors and the formulation of flame-retardant modification strategies.