Aiming at the problem of the density error in the large caliber gun test, the important parameters affecting the initial disturbance of the projectile are analyzed, and the influence of the projectile mass eccentricity on the firing accuracy of the gun is mainly studied. The coupling dynamics simulation model of projectile and barrel based on contact theory is established, and the disturbance caused by eccentric displacement, velocity and angular velocity of projectile is simulated numerically. The results show that the projectile mass eccentricity deviates from the bore axis is an important factor to cause the muzzle disturbance, and it is recommended that the mass eccentricity of the projectile be strictly controlled in the design of large caliber gun.