The invention relates to a special caliper for an armature keyway capable of measuring physical dimensions of the armature keyway, which comprises a main scale, an auxiliary scale, a display, depth measurement accessories, a dipstick, a scale frame and other parts, wherein the main scale consists of a main scale upper measuring jaw, a main scale lower measuring jaw and a main scale body; the main scale lower measuring jaw is short and takes the shape of a cuboid; the auxiliary scale consists of an auxiliary scale body, an auxiliary scale lower measuring jaw and an auxiliary upper measuring jaw; the auxiliary scale lower measuring jaw is long; the auxiliary scale, the scale frame and the depth measurement accessories are arranged on the main scale body and can all perform relative motion; the display and the dipstick are arranged on the auxiliary scale and move along with the auxiliary scale; a measuring surface and the bottom surface of the main scale lower measuring jaw contact the side face and the bottom surface of the keyway respectively; the auxiliary scale lower measuring jaw contacts an excircle of a shaft and measures the symmetry degree error of the keyway; a measuring surface of the main scale upper measuring jaw and a measuring surface of the auxiliary scale upper measuring jaw contact two side faces of the keyway respectively and measure the width of the keyway; the depth measurement accessories are placed on the shaft; and the dipstick contacts the bottom surface of the keyway and measures the depth of the keyway.