Based on the introduction of the jamming mechanism of the decoy for terminal-sensitive projectile, the analysis of the influence factors of the terminal-sensitive projectile hit probability and decoy jamming effectiveness, the model of the decoy jamming effectiveness evaluation is established. The concept of the falling rate of the hit probability and its expression are proposed. The falling rate of the hit probability concerns with the detectivity of the terminal-sensitive projectile, the decoy jamming characteristics and the operational area background characteristics. The test method of the decoy jamming effectiveness evaluation is discussed.