Based on the experimental perforation of projectile impacting stiffened plate, numerical simulations were conducted to study the failure pattern of target, projectile trajectory and the anti-perforation capability of target. The results show that stiffeners enhance the strength and rigidity of target, and change the whole structure capability and the failure pattern of target. For the single stiffened plate, single stiffened plate with "T" sectioned stiffener and cross stiffened plate, the increase of limit penetration velocity for different stiffeners and projectiles is 35%~80%. The maximum deflexion angle of projectile is 35°~40°.