A detachable pad fastening structure of a helmet includes a first fastening unit, which includes a first fastening member having a fastening hole formed therein and is disposed at an inner surface of a helmet main body. A second fastening unit includes a through-hole in the form of a slot in a detachable pad and a second fastening member formed to be movable relative to the through-hole, having at least a part protruding toward the outside of the detachable pad so as to be inserted into the fastening hole when the second fastening member is located at one side of the through-hole, and configured to be inserted toward the inside of the detachable pad so as to be removed from the fastening hole when the second fastening member is moved relative to the through-hole toward the other side of the through-hole.