In order to investigate excursion phenomena of the dispersion center of vertical target of an anti-aircraft gun, a series of test methods were adopted, such as fault representation, cause analysis, measure-taking and results verification. In firing tests, the projectile leaving instant from a muzzle was determined by measuring the floating displacement of a receiver, and the displacement curves of muzzle vibration were obtained with displacement sensors. Furthermore, the vertical target dispersion coordinates were detected with a graphic vertical target dispersion testing system, and the natural frequencies of muzzle vibration were acquired with frequency spectral analysis of the displacement curves of muzzle vibration. Based on the above, the effect of muzzle vibration on the dispersion laws of vertical target was analyzed. It was shown that as the nature frequencies of the gun structure are close to firing frequencies, the muzzle resonance appears, the projectile leaving instant from the muzzle is the same as that when the maximum displacement of muzzle vibration occurs, so the excursion phenomenon of the dispersion center of vertical target is observed. Through optimizing the structural stiffness, the phenomenon of muzzle resonance was eliminated. Finally, the excursion phenomenon of the dispersion center of vertical target basically disappeared.