A backward momentum generating device with coupled spring biased rod mechanism, being disposed through an internal wall settled between a housing and a stop wall of a toy gun, is disclosed. While a piston in the housing produces compressed air, a weight of the backward momentum generating device is pushed along a moving direction of the reciprocating motion of the piston, produces a reaction force, and results in an effect of a backward momentum applied to the housing of the toy gun. The coupled spring biased rod mechanisms include at least one front-section spring biased rod and at least one back-section spring biased rod, wherein the back-section spring biased rod is moveably disposed through a hollow channel formed in the weight of the toy gun.