The invention relates to a circumference measuring instrument which comprises Y shafts, a cable protective chain, a rotary motor, a workpiece positioning jig, a planetary speed reducer, an auger X shaft, a CCD camera, a displacement sensor, a carrier module and an operation table. During actual use, an operator fixes a workpiece to be tested through the workpiece positioning jig, then the carrier module drives the workpiece to move inward along one Y shaft, meanwhile, the CCD camera and the displacement sensor on the auger X shaft move to the position above the side, carrying the workpiece, of the carrier module, when the workpiece is right in the measurement range of the CCD camera and the displacement sensor, the rotary motor drives the workpiece to rotate, the workpiece starts to be measured, the carrier module on the Y shaft on the other side is moved to the front end of the operation table, the operator can conveniently place the workpiece in the measuring instrument, the steps are repeated, and the Y shafts on the two sides are used for measuring alternatively.