Abstract: In order to analyze the bad influence of the muzzle flow field on the motion of bullet in the exterior ballistic period, the motion characteristics of a 5.8 mm small-caliber bullet were simulated numerically based on the continuous medium hypothesis and the Realizable k-ε turbulence model for the muzzle flow field in after-effect period. Combining shadowgraph experimental results with numerical simulations, the velocity, force and dynamics were analyzed for the bullet in after-effect period, along with the underlying mechanisms of their changes. And the effect of varying in its charge and structure on bullet dynamical parameters was further explored. The results show that the bullet can move sequentially with acceleration, deceleration and near-uniform velocity in the after-effect period. The increase rate of the bullet velocity with the standard charge and shape is 0.183% in the after-effect period, and the axial force fluctuates repeatedly. When the charge is increased from 1.55 g to 1.75 g, the increase rate of the bullet velocity can remain around 0.200% in the after-effect period, and shorten the acceleration time by 0.012 ms. With the same charge, a sharper curved section and shorter tail structure can heighten the increase rate of the bullet velocity in the after-effect period and prolong the acceleration time.