Abstract: To obtain the ballistic deflection law of an elliptical cross-section projectile oblique penetration into concrete and reveal its internal mechanism, a systematic study was carried out by using numerical simulation. A reliable numerical simulation model was constructed. The oblique angle, attack angle and axis spin angle that affect the ballistic deflection were decoupling. Numerical simulations of the elliptical cross-section projectiles oblique penetration into concrete under different drop angles were carried out. The evolution laws of ballistic deflection and spin were deeply analyzed, and the mechanisms of ballistic deflection and spin were explained. The results show that the oblique angle and attack angle lead to the asymmetry of the force-bearing areas on the upper and lower surfaces of the projectile, and the attack angle also leads to the asymmetry of the surface stress of the projectile, eventually generating a deflection torque that prompts the deflection of the projectile. With the increase of the oblique angle and attack angle, the angular acceleration, angular velocity, attitude angle and ballistic offset of the projectile also show an increasing trend. In the case of oblique penetration with an oblique angle, the projectile in the upright position deflects slowly and for a long time, while the projectile in the lying position deflects quickly and for a short time. There is no absolute superiority or inferiority between the two in terms of ballistic stability. In the case of oblique penetration with an attack angle, the ballistic stability of the projectile in the upright position is better than that of the projectile in the lying position. The axis spin angle leads to the asymmetry of the projectile-target intersection condition. Besides offset and deflection, the projectile also has a self-rotating motion around the axis. When the axis spin angle increases from 0° to 90°, the projectile-target intersection condition undergoes a transformation from symmetry to asymmetry and then back to symmetry. The offset in the horizontal direction and the axis spin angle increment of the projectile first increase and then decrease. The research results can provide important references for the practical engineering application of the elliptical cross-section projectile.