<p>With the continuous expansion of the city’s high-voltage cable commissioning scale, cable failure caused by electrical fires is increasingly serious problem. As a critical insulating component, the thermal decomposition characteristics of the PVC outer sheath of high-voltage cables are essential for preventing cable fires. This study focuses on the YJLW02-64/110&#xa0;kV high-voltage power cable PVC sheath, aiming to investigate the impact of environmental humidity on its thermal decomposition and gas generation properties. Thermal decomposition experiments were conducted across ten humidity gradients, ranging from 10 to 100%, using a specially designed experimental platform that included heating, temperature and humidity control, and gas detection. Set a heat source of 200℃ to simulate the early fault state of high-voltage cables before a fire occurs. FTIR was employed to identify the functional groups in the thermal decomposition gases, while ion chromatography was used to quantify the HCl content. The results revealed that the thermal decomposition gases of the PVC sheath primarily contained H₂O, CO₂, CO, HCl, and low molecular olefins and alkanes. When the humidity was below 50% RH, characteristic absorption peaks corresponding to alcohols (C–OH) and aryl ring/ketones (C–C) were observed in the infrared spectrum. The absorbance of C–H, C–C, and C–OH functional groups increased as humidity decreased, while CO₂ absorbance peaked and then decreased. In contrast, HCl production increased by more than a thousandfold. The article reveals the influence of humidity on the gas generation characteristics of the thermal decomposition of the PVC outer sheath of high-voltage cables, which can provide theoretical support for the early warning technology of high-voltage cable fires.</p>