A strap fastening device includes two fastening straps and a tightness adjusting unit. The fastening straps each have a fixing end and an opposing free end. The tightness adjusting unit includes an assembling box and two locating members. The assembling box is provided at a left and a right half with at least one non-return retaining tooth each. The two locating members are slidably mounted to the assembling box and connected to the free ends of the two fastening straps, such that the free ends of the fastening straps are not turned back to expose from the assembling box. The slidable locating members respectively have a raised locating rib releasably engaging with the at least one non-return retaining tooth at the left and the right half of the assembling box. The two locating members can be conveniently slid with one hand to adjust the tightness of the fastening straps.