When the projectile was launched by a barrel weapon, the precursor flow field had greater impact on the development of combustion gas flow of subsequent propellant and the movement of projectile. To analyze the effect of precursor flow field on the structure of combustion gas jet and movement of projectiles, based on the finite volume method, two different 2-d axisymmetric numerical models with and without the precursor flow field were established using the Whole Zone Movement Method for block grid division and Realizable k-ε turbulence Model and coupling the interior ballistic process and the Six-DOF equations. Taking the 300 mm counter-mass propelling gun as example, the muzzle flow field characteristics at the ejection speed of 1730 m·s^-1 were studied. The results show that when there is no precursor flow field, the wavefront of propellant combustion gas is approximately spherical, and the propellant combustion gas is unable to catch up with the projectile. But under the interference of precursor flow field, the ejectingn velosity of propellant combustion gas increases by about 200 m·s^-1, and the combustion gas catches up with and surrounds the projectile, the maximum pressure in the flow field decreases by a half, the temperature increases by more than 1000 K and the projectile base pressure decreases by about 1.3 MPa.