The invention discloses a sphere diameter measuring instrument including a base, a plurality of measuring anvils, and a positioning member, one measuring anvil is fastened on the base through a fastener, a sphere to be measured is arranged on the measuring anvil through the positioning member, a support part with an extension arm is arranged on the base, the extension arm extends over the measuring anvil, the extension arm and the support part form an L-shaped support frame, the extension arm is provided with a vertical through hole, a limiting sleeve is fixedly arranged in the through hole, a transition plane measuring head is arranged in the limiting sleeve, the lower end of the transition plane measuring head extends out of the limiting sleeve, a limiting device for limiting the vertical movement distance of the transition plane measuring head is arranged between the transition plane measuring head and the limiting sleeve, a dial gauge is arranged on the extension arm through an installing seat, the measuring head at the lower end of the dial gauge extends into the through hole, and then abuts against the upper end of the transition plane measuring head, a resetting device is arranged in the through hole, and a lifting device is arranged on the support frame. The sphere diameter measuring instrument has the advantages of low use cost, high detection precision, and high repeatability.