The invention discloses a fire damper for a secondary detonation-breathing pulse detonation engine. The fire damper is in a double-ring structure formed by an outer ring of a front disk, an outer ring of a rear disk and a plurality of same triangular prisms in the same inner rings. The fire damper is fixed at one end of a pre-detonation tube, located on a lip of an ejector. An air channel is formed between the fire damper and the lip of the ejector. The front disk and the rear disk are staggered. The triangular prisms of the rear disk are located among the triangular prisms of the front disk. Air flowing in from the front end of the front disk contacts with edges of the triangular prisms and flows by the front disk, an ejection air channel between each two triangular prisms of the front disk is exactly blocked by the triangular prisms of the rear disk, and accordingly forward flow resistance is smaller than reverse flow resistance. The fire damper for the secondary detonation-breathing pulse detonation engine is effective in preventing forward transmission of secondary denotation and capable of ejecting mass air, operation of the engine is more stable, and thrust of the pulse detonation engine is higher.