A fastening device (100, 100a), comprising: a case unit (200, 200a), having an axial direction; a knob (500, 500a), covered on the case unit (200, 200a) along the axial direction; an engaging unit (400, 400a), coupled to the knob (500, 500a) and comprising an annular body and a pawl arm (450, 450a), wherein the pawl arm (450, 450a) has a proximal end and a distal end, and the proximal end is connected to the annular body; and a spool (300, 300a), configured for a lace to wind therearound; wherein: the engaging unit (400, 400a) further comprising an engaging portion (460, 460a), the engaging portion (460, 460a) being located on the pawl arm (450, 450a) and protruding outward along a radial direction of the case unit (200, 200a), wherein a first distance D 1 is contained between the proximal end and the distal end, a second distance D 2 is contained between the engaging portion (460, 460a) and the proximal end, and a relationship of D 2 ≤ (2D 1 / 3) is satisfied; wherein one of the knob (500, 500a) and the case unit (200, 200a) comprising a plurality of mounting teeth (230, 230a), the pawl arm (450, 450a) is selectively engaged with at least one of the mounting teeth (230, 230a), the engaging unit (400, 400a) is driven by the knob (500, 500a) to switch between a first position and a second position along the axial direction, when the engaging unit (400, 400a) is in the first position, the engaging unit (400, 400a) prohibits the spool (300, 300a) from releasing the lace, and when the engaging unit (400, 400a) is in the second position, the engaging portion (460, 460a) is engaged with at least one of the mounting teeth (230, 230a), and the engaging unit (400, 400a) does not prohibit the spool (300, 300a) from releasing the lace. a case unit (200, 200a), having an axial direction; a knob (500, 500a), covered on the case unit (200, 200a) along the axial direction; an engaging unit (400, 400a), coupled to the knob (500, 500a) and comprising an annular body and a pawl arm (450, 450a), wherein the pawl arm (450, 450a) has a proximal end and a distal end, and the proximal end is connected to the annular body; and a spool (300, 300a), configured for a lace to wind therearound; wherein: the engaging unit (400, 400a) further comprising an engaging portion (460, 460a), the engaging portion (460, 460a) being located on the pawl arm (450, 450a) and protruding outward along a radial direction of the case unit (200, 200a), wherein a first distance D 1 is contained between the proximal end and the distal end, a second distance D 2 is contained between the engaging portion (460, 460a) and the proximal end, and a relationship of D 2 ≤ (2D 1 / 3) is satisfied; wherein one of the knob (500, 500a) and the case unit (200, 200a) comprising a plurality of mounting teeth (230, 230a), the pawl arm (450, 450a) is selectively engaged with at least one of the mounting teeth (230, 230a), the engaging unit (400, 400a) is driven by the knob (500, 500a) to switch between a first position and a second position along the axial direction, when the engaging unit (400, 400a) is in the first position, the engaging unit (400, 400a) prohibits the spool (300, 300a) from releasing the lace, and when the engaging unit (400, 400a) is in the second position, the engaging portion (460, 460a) is engaged with at least one of the mounting teeth (230, 230a), and the engaging unit (400, 400a) does not prohibit the spool (300, 300a) from releasing the lace. the engaging unit (400, 400a) further comprising an engaging portion (460, 460a), the engaging portion (460, 460a) being located on the pawl arm (450, 450a) and protruding outward along a radial direction of the case unit (200, 200a), wherein a first distance D 1 is contained between the proximal end and the distal end, a second distance D 2 is contained between the engaging portion (460, 460a) and the proximal end, and a relationship of D 2 ≤ (2D 1 / 3) is satisfied; wherein one of the knob (500, 500a) and the case unit (200, 200a) comprising a plurality of mounting teeth (230, 230a), the pawl arm (450, 450a) is selectively engaged with at least one of the mounting teeth (230, 230a), the engaging unit (400, 400a) is driven by the knob (500, 500a) to switch between a first position and a second position along the axial direction, when the engaging unit (400, 400a) is in the first position, the engaging unit (400, 400a) prohibits the spool (300, 300a) from releasing the lace, and when the engaging unit (400, 400a) is in the second position, the engaging portion (460, 460a) is engaged with at least one of the mounting teeth (230, 230a), and the engaging unit (400, 400a) does not prohibit the spool (300, 300a) from releasing the lace.