A fastening device includes a main part and a fastening member. The main part is composed of a base, a spool, a knob and an engaging unit. The fastening member disposed at the main part. The spool is disposed at the base. The knob disposed at the base includes a spiral track. The engaging unit coupled to the spool includes a guiding portion coupled to the spiral track. The guiding portion is limitedly moved along the spiral track by rotating the knob such that the engaging unit has a first position and a second position relative to the knob. The engaging unit prohibits the spool from rotating toward a releasing direction when the engaging unit is at the first position, and the engaging unit allows the spool to be rotated toward the releasing direction when the engaging unit is at the second position.