In order to quantitatively describe factors causing trajectory deflection in the process of projectiles penetrating concrete targets, the model was adopted, which was random aggregate of concrete targets penetrated by rigid projectiles. Trajectory deflection was taken as the index, and the striking velocity varied from 300 m/s to 1 200 m/s and four diameter ratios 0.40, 0.85, 1.67, 2.67 as the factor, the influence of the two factors on trajectory deflection was researched. The results show that during projectile penetrating, the deflection angle decreases as the diameter ratio increases. For the projectiles penetrating concrete targets, when the diameter ratio is less than 2, the trajectory deflection angle is large; For low speed (less than 500 m/s) penetration, the trajectory deflection angle fluctuation is large, and the high-speed (not less than 800m/s) penetration, the trajectory deflection angle fluctuation is small. and when the diameter ratio is greater than 2, the trajectory deflection angle is small In the process of penetration, as for the the fluctuation. It's show that for the projectiles whose diameter ratio is less than 2, the concrete micro-mechanical structure promotes the trajectory deflection's development vastly, and the trajectory deflection is affected significantly by striking velocity. For the projectiles whose diameter ratio isn't less than 2, both the concrete micro-mechanical structure and the striking velocity don't affect the trajectory almost.