The invention discloses a stepless diameter-alterable roller and a diameter-changing method thereof and belongs to the technical field of winding rollers. The stepless diameter-alterable roller comprises a roller core, supporting frames, supporting roller bodies, a lead screw nut mechanism and a driving mechanism; each supporting frame comprises a supporting roller body, the two ends of each supporting roller body are connected with the corresponding supporting frame, and each of the two ends of the roller core is fixedly provided with one supporting frame; the lead screw nut mechanism is arranged on the roller core in a sleeving mode, the two ends of the lead screw nut mechanism are connected with the supporting frames, and the lead screw nut mechanism is driven by the driving mechanism to move so that the trouble of diameter adjusting of the supporting roller bodies through manual operation can be avoided; and through matching of the driving mechanism and the lead screw nut mechanism, the time and the labor are saved during adjusting. Aiming at the problem that in the prior art, the external-diameter-alterable winding rollers are difficult to adjust, the lead screw nut mechanism is adopted, a motor can automatically control a bi-directional overrun clutch to transfer power for enabling a lead screw to rotate, and through computing control of a numerical control system, the external diameters of the winding rollers can be continuously and accurately adjusted.