The invention provides a tool for centering the circle center of a workpiece. The tool comprises an expansion plate, a dial gage, a rotary disk and a clamping stem, wherein one end of the rotary diskis movably connected with the expansion plate provided with the dial gage; the other end of the rotary disk is movably connected with the clamping stem; the clamping stem is connected with a machine tool rotary shaft. The tool can conduct adjustment operation on workpieces with different diameters through the expansion function of the expansion plate. Meanwhile, the expansion plate can rotate flexibly with the clamping center as the center to complete the circle center centering. Through the tool, the circle center of the workpiece can be centered conveniently, quickly and accurately, and themachining efficiency and quality of products are improved.