Abstract: An adjusting device for straps of a knapsack is disclosed. The straps each have an upper band provided at an upper end and a lower band provided at a lower end. The adjusting device includes a back board attached to a back surface of the knapsack, the back board having two longitudinal slots parallel to each other. The back board has two openings, of which a width is larger than a width of the sliding blocks, and respectively defined at lower ends of the longitudinal slots. A sliding board is attached to the straps and has multiple sliding blocks movably mounted in the corresponding longitudinal slots. The sliding board has necks, of which a width is smaller than a width of the longitudinal slots, and respectively formed between the sliding blocks and the sliding board and received in the corresponding longitudinal slots.