A string tension adjustment structure includes a dock, a winding axle longitudinally coupling with the dock that has an upper end fastened to a worm gear held in the dock and a lower end to wind a string, and a transverse rotary knob running through the dock that has a worm at one end to engage with the worm gear. The dock has a pair of insertion troughs to hold a U-shaped elastic reed. The elastic reed has a pair of clipping blades each has a notch at the top portion to clip an indented neck formed on the worm to anchor the rotary knob on the dock. The structure is simpler and fabrication and assembly are easier.