The invention relates to an online wheel run-out detecting device. The online wheel run-out detecting device includes a frame, a chassis, a lifting cylinder, supports, bearing bases, linear bearings, mounting plates, guide shafts, a lifting shaft, a servo motor, a synchronous pulley, a connecting plate, a synchronous belt, a synchronous pulley, a base, a connecting shaft, a servo motor, a shaft sleeve, a lower end cap, a connecting shaft, a shaft sleeve, and an oil cylinder. The online wheel run-out detecting device can meet requirements of online wheel run-out detection, meanwhile it has the characteristics of simple structure, convenience in manufacturing, stable performance and capability of meeting machining requirements on precision, and can meet requirements of automatic production.