The invention relates to a special measuring tool, in particular to a special measuring tool for measuring the symmetry degree of a shaft part key groove and solve the technical problem that people can measure the symmetry degree of the shaft part key groove conveniently, a manual ruler is not needed, and the phenomenon that the efficiency is affected by hand pain is avoided. The technical schemeof the special measuring tool for measuring the symmetry degree of the shaft part key groove comprises a hollow handle, a first support bar, a support plate, an adjusting device, a first measuring rodand the like; the first support rod is fixedly connected to the lower part of the left side of the front side of the hollow handle, and a first gear is rotationally arranged at the upper part of therear side of the first support rod. According to the special measuring tool for measuring the symmetry degree of the shaft part key groove, under the action of the first measuring rod and the second measuring rod, a press block is pushed, the first measuring rod and the second measuring rod can measure the symmetry degree of the shaft part key groove, the manual ruler is not needed to measure theposition length of the key groove to obtain the symmetry degree, the operation is more convenient, and a detection shaft can move more smoothly in the hollow handle under the action of a universal ball.