The invention relates to an anti-seepage fan-shaped ultrafine atomized denitration spray gun for metal sealing and belongs to the technical field of denitration. The anti-seepage fan-shaped ultrafineatomized denitration spray gun for metal sealing comprises a gas-liquid mixer, a compressed air spherical outer thread joint, an ammoniacal liquor spherical outer thread joint, a gas-liquid primary expansion cavity, a cold air suction inlet, a protecting sleeve, a gas-liquid secondary expansion cavity, a high-temperature gas phase conversion cavity and a wide angle horizontal atomizing nozzle. A compressed air source metal hose is connected to the compressed air spherical outer thread joint, an ammoniacal liquor hose is connected to the ammoniacal liquor spherical outer thread joint, the compressed air spherical outer thread joint and the ammoniacal liquor spherical outer thread joint are in threaded connection by means of a thread to form the gas-liquid mixer, the gas-liquid primary expansion cavity is formed in the gas-liquid mixer, the gas-liquid secondary expansion cavity and the high temperature gas phase conversion cavity form a reducing pipe fitting of an integrated structure, one end of the reducing pipe fitting is in threaded connection to the gas-liquid mixer while the other end of the reducing pipe fitting is in threaded connection to the wide angle horizontal atomizingnozzle, and the cold air suction inlet communicates to the protecting sleeve. The anti-seepage fan-shaped ultrafine atomized denitration spray gun for metal sealing provided by the invention has the advantages that the atomizing effect is improved, the denitration efficiency is improved, and the ammonia consumption is reduced.