Radioactive iodine is one of the typical nuclear fission products. As a result, adsorption-separation- solidification of radioactive iodine (¹²⁹I, ¹³¹I, etc.) is important for nuclear power operations and spent fuel reprocessing. In this study, a novel type of bismuth-based composite nanofiber membrane (Bi@SiOCNF) was prepared by thermal reduction and electro-spinning technology using a kind of commercial poly-thylsilsesquioxane resin (MK resin) as raw material. Based on SiOC fiber, the bismuth metal was uniformly loaded on the surface of SiOCNF and in the three-dimensional network space, showing excellent capability for capturing and immobilizing gaseous iodine. According to the adsorption experiment results, the material can reach the maximum saturated adsorption capacity as high as 515.2 mg/g within 2 h. Adsorption mechanism of gaseous iodine on bismuth-based SiOCNF composite nanofiber membrane was through chemical adsorption and physical adsorption which was verified by XRD, XPS and other tests. Thermogravimetry analysis (TGA) demonstrated that the Bi@SiOCNF owned an excellent thermal stability. All above properties indicate that the material has potential applications in the capturing, fixation and storage of radioactive gaseous iodine in nuclear power plants and spent fuel reprocessing plants.