To analyze the muzzle flow field of hyper-velocity projectile, we established a 2-D axisymmetric numerical simulation model based on the finite volume method. The Navier-Stokes equations are solved on an extended mesh covering the whole computational domain. The holistic movement of a partitioned mesh processing method and realizable k-epsilon turbulence model is used coupling the process of interior ballistic and the Six-DOF UDF program which controls the projectile moving. Taking the 300 mm balanced gun as an example, the simulation results indicate here that the initial flow field has been formed clearly when the projectile is 3.3 m away from muzzle, and the pressure is about 3.8 MPa when the shock wave gets to the muzzle. It is noteworthy that the propellant gas velocity reaches 2 500 m/s but it is still unable to catch up and surround the projectile. So the impact of muzzle flow on the hyper-velocity projectile movement is different from the impact on the low-speed projectile. Subsequently it is formed a completely wave system composed of the multilayer shock wave including coronary blast, shock wave, reflected shock wave, Mach disk and the discontinuity surface, which makes the wave structure and the jet flow field more obvious.