Cattail fluff is widely distributed in nature. Scanning electron microscopy revealed it has a curly flexible characteristic with groove structure. Novel flexible composite fibers with good conductivity were synthesized by using cattail fluff as substrate via in situ polymerization. Aniline monomers were absorbed and polymerized on the cattail fluff fibers. The conductivities of the composite fibers increase with the increasing aniline concentration. This method is very simple and effective. These flexible conductive composite fibers may find potential applications in flexible electronic devices.