The invention provides an auxiliary measuring device of an inner bore ring slot. The auxiliary measuring device comprises a measuring head which comprises two measuring slide blocks, wherein the measuring slide blocks are assembled on a positioning barrel in a sliding manner along the radial direction; the positioning barrel is provided with an outer peripheral surface fitted with the inner peripheral surface of the inner bore of the measured piece. The auxiliary measuring device of the inner bore ring slot disclosed by the invention has an unmeasured state and a measured state, wherein under an unmeasured state, the measuring slide blocks completely retract in the positioning barrel, and the outer ends of the measuring slide blocks are not exposed of the outer peripheral surface of the positioning barrel; under the measured state, the measuring slide blocks extend out of the outer peripheral surface of the positioning barrel, and the outer ends of the measuring slide blocks are in thrust limit fit in the outward direction of the slot bottom of an inner ring slot. The auxiliary measuring device of the inner bore ring slot disclosed by the invention can be used for guaranteeing that the measuring head is vertical to the inner bore axis of the measured piece, is accurate in measured data, very simple in structure and relatively low in using cost.