An under-protector fastening structure includes a bumper assembly coupled to a chassis frame, wherein the bumper assembly includes a bumper inner member and a bumper cover coupled to an outer side of the bumper inner member, and an under-protector coupled to the bumper inner member of the bumper assembly in a direction vertical to a length direction of a vehicle body, wherein the under-protector includes an under-protector mounting assembly coupled to the bumper inner member and an under-protector panel coupled to the under-protector mounting assembly, and wherein the under-protector mounting assembly includes a pipe member extending in the direction vertical to the length direction of the vehicle body, a mounting bracket coupled to the pipe member and the bumper inner member, and an under-protector fixing bracket having a first side coupled to the mounting bracket and a second side coupled to the under-protector panel.