Accurate and effective monitoring of SF6 decomposition components inside the gas-insulated equipment （GIE） is crucial for the equipment fault diagnosis and condition assessment. However，the existing external detection methods are limited by sampling points and the diffusion rate of these components，making it difficult to accurately determine the concentration of the components. Therefore，in this paper，a method for detecting the SF6 decomposition component SO2 based on surface-coated micro/nano fibers （MNF） is proposed，which has the potential to be applied in built-in fiber optic sensing for GIE online monitoring. The optical gas-sensitive response performance of the graphene is analyzed based on the density functional theory. A fiber-loop ring-down （FLRD） gas detection system is constructed，and typical SF6 decomposition component detection experiments are carried out. The simulation results show that the optical properties of graphene change significantly after the adsorption of SO2 gas molecules，indicating its excellent optical gas-sensitive response performance to SO2. The experimental results also show that the gas detection system can detect the trace SO2 gas at room temperature. It shows a good linear relationship within the concentration range of 0~200 μL/L. The maximum error of SO2 detection is 4.89%，and the sensitivity is 0.81 ns/（μL·L–1）. The detection performance is improved by adding an optical fiber amplifier，and the sensitivity of the system reaches 1.24 ns/（μL·L–1）. The method proposed in this paper provides a new approach for online monitoring of SF6 decomposition components using GIE built-in optical fiber sensors.