The knowledge of CFD and dynamic mesh were applied to establish three-dimensional model for a parallel-firing cluster weapon and the N-S equation was calculated to simulate the structure of muzzle flow field influenced by high-speed projectile. In the simulation, the development of flow field and the projectile movement are mutual; The projectile side force produced by the complicated flow field is the main reason of the drop of firing precision. In the case of the same pitch, the largest side velocity of side projectiles (including muzzle brake) is the two-thirds of the one without it.