An on-line wheel airtightness detecting device where a centering device centers a wheel above a roller bed, and a sixth cylinder causes arms to drive clamping jaws through a second gear, second racks, and fifth guide rails to clamp the wheel; a servo motor turns the wheel 90°; a fifth cylinder drives the wheel to a support roller through a fourth guide rail; a second cylinder drives the wheel through first guide pillars; a first cylinder drives the wheel through a first guide rail and flush with a right rubber disc; the hydraulic cylinder drives a left rubber disc through second guide pillars; a motor drives a hollow shaft and the wheel to rotate through a small pulley, a large pulley and a synchronous belt; and compressed air is charged into the hollow shaft, to detect air tightness.