Abstract: An adjusting strap structure provided for a pair of swim fins each of which includes a foot pocket having two side walls each defining a first hole and a second hole. The adjusting strap structure includes an adjusting strap having two opposite sides and two distal ends, two first snapping members each mounted on the respective distal end of a first side of the adjusting strap, and a second snapping member mounted on the first side of the adjusting strap for detachably engaging with the two first snapping members. In such a manner, each distal end of the adjusting strap extends through a respective side wall of the foot pocket, and each first snapping member is detachably engaged with the second snapping member. In addition, each distal end of the adjusting strap in turn passes through the foot pocket into the second hole to extend outward from the second hole, and then passes through the first hole into the foot pocket to extend outward from the foot pocket.