A headband adjusting device includes a mounting base including an internal ratchet, a linkage mounted on the mounting base. The linkage includes a transmission member and an elastic position limiting member configured to abut against a tooth of the internal ratchet. The headband adjusting device also includes a thumbwheel connected with the linkage and configured to cause the linkage to rotate, the thumbwheel including a disengaging member corresponding to the position limiting member. When the thumbwheel rotates in a first direction, the disengaging member abuts against the position limiting member to unlock the internal ratchet. When the thumbwheel rotates in a second direction, the position limiting member rotates while contacting the internal ratchet. The headband adjusting device further includes a transmission mechanism coupled with the transmission member. The linkage is configured to drive the transmission mechanism, which drives a headband to extend or retract.