Abstract: A fastening ring structure includes a metallic strap formed with a U-shaped bending section spaced from one end of the strap by a predetermined distance. Two ends of the bending section are formed with a first and a second sections. The second section is formed with multiple engaging holes at equal intervals. Two inner engaging hooks project from inner side of the first section toward the bending section. Two outer engaging hooks and a locating plate project from outer side of the first section toward the bending section. Two ends of the strap are curved into a circular shape with the first section overlapping the second section on inner or outer side thereof. The inner or outer engaging hooks and locating plate of the first section are engaged in the engaging holes of the second section to form a fastening ring. The inner or outer engaging hooks can be engaged in different engaging holes to achieve a longer adjustment length of the fastening ring.