The invention relates to the technical field of environmental protection, in particular to an ammonia spraying device and a flue gas denitration device. The ammonia spraying device comprises an ammonia spraying pipeline suitable for being horizontally arranged in a flue, the cross section of the ammonia spraying pipeline being rectangular, the side, perpendicular to the flowing direction of flue gas in the flue, of the cross section of the ammonia spraying pipeline being a short side, the side parallel to the flowing direction of flue gas in the flue being a long side, and the surface, facing the flowing direction of the flue gas, of the ammonia spraying pipeline being a windward side; a flow guide part arranged on the windward side of the ammonia spraying pipeline; and a nozzle arranged on the ammonia spraying pipeline. On the premise that the length of the ammonia spraying pipeline is not changed, the projection area of the ammonia spraying pipeline can be greatly reduced, the blocking area for flue gas is reduced, the deamination efficiency of the system is ensured, the resistance of the system is effectively reduced, the mass transfer path and area are increased, and the deamination efficiency is improved. The flow guide part is arranged on the windward side of the ammonia spraying pipeline, so that the flowing of flue gas can be guided, the flue gas is prevented from directly colliding with the windward side of the ammonia spraying pipeline, and the abrasion of the windward side can be reduced.