The invention relates to equipment belonging to the field of converter steelmaking, and in particular relates to an oxygen gun opening nitrogen sealing device. The nitrogen sealing device comprises a nitrogen sealing plug and a nitrogen sealing base, wherein the nitrogen sealing plug comprises an upper big-port end and a lower small-port end; matched sealing surfaces are formed by the outer surface of the nitrogen sealing plug and the inner surface of the nitrogen sealing base; an oxygen gun opening is formed in the nitrogen sealing base; an oxygen gun is sleeved with the nitrogen sealing plug; a nitrogen sealing round hole I is formed in the lower part of the nitrogen sealing plug; the nitrogen sealing round hole I is communicated with a plurality of nitrogen channels I; a nitrogen sealing round hole II is formed in the upper part of the nitrogen sealing plug; the nitrogen sealing round hole II is communicated with a plurality of nitrogen channels II; a conical cavity is formed in the nitrogen sealing plug. The nitrogen sealing device disclosed by the invention has the advantages of being capable of sealing twice and decompressing high-pressure smoke, and good in sealing effect.