A seatbelt adjusting device includes: a guiding track provided with a plurality of first engaging members; a belt-connecting slider mounted on the guiding track and slidable on the guiding track; an actuator mounted movably on the belt-connecting slider, provided with a second engaging member, and operable to move relative to the belt-connecting slider between an engaging position, in which the second engaging member engages releasably an aligned one of the first engaging members, and a disengaging position, in which the second engaging member is disengaged from the first engaging members; and an urging member urging against the actuator for restoring the actuator from the disengaging position to the engaging position.