A webbing adjustment device has a cover, a frame, a slider, and a guiding ring member. The frame is up and down moveably disposed in the cover. The slider is located in the cover, is moveably disposed in a body of the frame, and has a through slot formed through the slider. Webbing inserted through the through slot is locked or unlocked by a relative motion between the frame and the slider. The guiding ring member is detachably disposed on the cover, and has a guiding ring and a positioning element connected to the guiding ring. The webbing is inserted through the guiding ring. The cover is detachably hung on the guiding ring. The webbing adjustment device can lock the webbing automatically by the tension of the webbing.