Compared with the traditional gunpowder bullets, electromagnetic launch bullets have the advantages of high muzzle velocity and long fire range. However, the bullet shape is no longer axisymmetric because of the armature groove at the tail, resulting in unique aerodynamic characteristics. Based on the three-dimensional unsteady Navier-Stokers equation, the aerodynamic characteristics of electromagnetic projectile were analyzed by using the sliding grid technique. Results show that for the hypervelocity spinning electromagnetic projectile, the Magnus effect comes from the interaction of shock layer distortion and the upwind area variation of the armature arm. The aerodynamic force and the moment varies periodically with the roll angle due to the periodic variation of the upwind area of the armature arm, and the Magnus moment reaches the minimum and the maximum at the roll angle of 45° and 135°, respectively. The influence of the armature arm groove is remarkable, which not only worsens the Magnus effect (increased by more than 50% at 135°), but also makes the pressure center move forward periodically (the absolute forward amount is up to 5%). Furthermore, with the increase of spinning speed, the increase of the Magnus moment and the pressure center move-forward effect become more and more significant, which is harmful to the dynamic stability of the projectile.