The invention provides a capping device. The capping device is applied to a steel ladle tundish and comprises a driving component and a capping swinging arm mechanism, wherein the capping swinging armmechanism comprises force application components and driving arms; one ends of the force application components are connected with the driving component and the other ends of the force application components make contact with one ends of first group driving arms through limiting blocks; sealing caps are mounted at the other ends of the driving arms and the sealing caps are used for sealing pouring holes of the tundish; therefore, when the pouring holes of the tundish need to be capped and sealed, the pouring holes can be sealed in the way of using the driving component to act on the force application components, driving the driving arms to move simultaneously by the force application components and placing the sealing caps on the pouring holes of the tundish by the driving arms; the pouring holes of the tundish are sealed during the course of pouring, so that the tundish can maintain an oxidizing atmosphere, molten steel can be prevented from oxidation and the quality of steel can beguaranteed.