The invention discloses a device for accurately measuring the shear blade overlapping amount of an edge-trimming disk shear for a magnesium plate and is applicable to a mechanism for accurately measuring the shear blade overlapping amount of the edge-trimming disk shear for the magnesium plate. The device comprises a high-accuracy displacement sensor, a sensor stabilizing device, a displacement detection magnetic ring, an ejector rod mechanism, a pressing sleeve, a pressing spring, a vertical cycloidal pin gear speed reducer, a shaft-mounted worm speed reducer, a gear shaft, a gear ring, an eccentric sleeve and a disk shear shaft, wherein the vertical cycloidal pin gear speed reducer is connected with the shaft-mounted worm speed reducer; the gear shaft is arranged in the shaft-mounted worm speed reducer; the gear ring is meshed with the gear shaft; the gear ring and the eccentric sleeve are arranged together; the disk shear shaft is assembled in the eccentric sleeve; the ejector rod mechanism is arranged in the pressing sleeve and contacts the eccentric sleeve through the pressing spring; the detection magnetic ring is arranged on the ejector rod mechanism. The device is advanced in structural form, accurate in measurement value and convenient to install and maintain; the influence of deviation in machining and manufacturing processes can be fundamentally eliminated.