The fastening structure includes an inner sleeve, a quick-release handle and an outer sleeve. The inner sleeve has an outer threaded conic surface, a through slot, shrinkable slots and claw plates between adjacent two of the shrinkable slots. The quick-release handle is provided at a bottom end of the inner sleeve for controlling a width of the through slot. The outer sleeve is used for connecting the footrest ring and has an inner threaded conic surface to screw the outer sleeve onto the inner sleeve.