The invention discloses a variable-diameter plunger. The variable-diameter plunger comprises a spring seat, a reset spring, a first limiting joint, a second limiting joint, a plunger pipe, a central pipe, flake-like springs, a valve seat and a poking rod. The spring seat is connected with the lower end of a fishing head; the reset spring is fixed to the spring seat; the first limiting joint can jack the reset spring; the plunger pipe is located between the first limiting joint and the second limiting joint and formed by enclosing of more than two plunger sections, wherein the upper end faces of the plunger sections are located on the inner side of the first limiting joint, the lower end faces of the plunger sections are located on the inner side of the second limiting joint, and the plunger sections can tend to get close to one another or be separated from one another; the central pipe is located in the plunger pipe, the upper end of the central pipe is connected with the small-diameter position of the first limiting joint, and the lower end of the central pipe is connected with the small-diameter position of the second limiting joint; the flake-like springs correspond to the plunger sections and are arranged in the axial direction of the central pipe, the middle parts of the flake-like springs are fixed to the side wall of the central pipe, and the two ends of the flake-like springs jack the inner walls of the plunger sections; the valve seat is arranged at the lower end of the second limiting joint and provided with a valve ball; and the poking rod penetrates through thereset spring, the first limiting joint, the central pipe and the second limiting joint, the upper end of the poking rod is fixed to the spring seat, and the lower end of the poking rod can jack the valve ball. The diameter of the plunger can be changed.