A fastening device (100, 120), comprising a main part and a fastening member, the main part composed of a base (200, 200b), a spool (300, 300b), a knob (500, 500b) and an engaging unit (400, 400b), the fastening member disposed at the main part such that the base (200, 200b), the spool (300, 300b), the knob (500, 500b) and the engaging unit (400, 400b) are assembled, wherein: the spool (300, 300b) is disposed at the base (200, 200b), and a lace is wound around the spool (300, 300b); the knob (500, 500b) is disposed at the base (200, 200b), and the knob (500, 500b) comprises a spiral track (530, 530b); the engaging unit (400, 400b) is coupled to the spool (300, 300b), and the engaging unit (400, 400b) comprises a guiding portion (430, 432b) coupled to the spiral track (530, 530b); and the guiding portion (430, 432b) is limitedly moved along the spiral track (530, 530b) by rotating the knob (500, 500b), such that the engaging unit (400, 400b) has a first position and a second position relative to the knob (500, 500b), the engaging unit (400, 400b) prohibits the spool (300, 300b) from rotating toward a releasing direction when the engaging unit (400, 400b) is at the first position, and the engaging unit (400, 400b) allows the spool (300, 300b) to be rotated toward the releasing direction when the engaging unit (400, 400b) is at the second position; characterized in that, the engaging unit (400, 400b) further comprises a pawl arm (470, 470b), the base (200, 200b) comprises a plurality of inner teeth (230, 230b), when the engaging unit (400, 400b) is at the first position, the pawl arm (470, 470b) is engaged with the inner teeth (230, 230b) in the releasing direction to prohibit the spool (300, 300b) from rotating, and the pawl arm (470, 470b) is disengaged with the inner teeth (230, 230b) in a tightening direction opposite to the releasing direction. the spool (300, 300b) is disposed at the base (200, 200b), and a lace is wound around the spool (300, 300b); the knob (500, 500b) is disposed at the base (200, 200b), and the knob (500, 500b) comprises a spiral track (530, 530b); the engaging unit (400, 400b) is coupled to the spool (300, 300b), and the engaging unit (400, 400b) comprises a guiding portion (430, 432b) coupled to the spiral track (530, 530b); and the guiding portion (430, 432b) is limitedly moved along the spiral track (530, 530b) by rotating the knob (500, 500b), such that the engaging unit (400, 400b) has a first position and a second position relative to the knob (500, 500b), the engaging unit (400, 400b) prohibits the spool (300, 300b) from rotating toward a releasing direction when the engaging unit (400, 400b) is at the first position, and the engaging unit (400, 400b) allows the spool (300, 300b) to be rotated toward the releasing direction when the engaging unit (400, 400b) is at the second position; characterized in that, the engaging unit (400, 400b) further comprises a pawl arm (470, 470b), the base (200, 200b) comprises a plurality of inner teeth (230, 230b), when the engaging unit (400, 400b) is at the first position, the pawl arm (470, 470b) is engaged with the inner teeth (230, 230b) in the releasing direction to prohibit the spool (300, 300b) from rotating, and the pawl arm (470, 470b) is disengaged with the inner teeth (230, 230b) in a tightening direction opposite to the releasing direction.