A fastening device includes a fixing bracket and a fastener. The fixing bracket includes a fixed arm and two adjusting arms respectively connecting at two ends of the fixed arm. Each of the two ends of the fixed arm defines a plurality of spaced retaining slots. The retaining slots are located at different positions along the fixed arm. Each of the two adjusting arms has a sliding end retained selectively in one of the retaining slots and an opposite fixing end. The fastener extends through the fixing end of each of the two adjusting arms.