To investigate the influence of truncated nose diameter on the penetrating capability of semi-armor-piercing projectiles for air-to-ship missiles, this work established five semi-armor-piercing projectiles with different truncated nose diameters. Utilizing LS-DYNA software, numerical simulations were conducted on five nose-shaped projectiles penetrating a 15.2 mm thick 921A thin steel plate at a targeting velocity of 606 m/s. The analysis included examining the effects of the truncated nose diameter and impact on velocity loss variations upon impact. Results indicate that, during vertical penetration, the percentage of velocity loss increases with the increase in truncated nose diameter, with the flat-nosed projectile exhibiting the highest velocity loss percentage at 9.6%. Meanwhile, the velocity loss of the projectile increases with the increase of the diameter of the truncated nose under the oblique penetration condition. The percentage of velocity loss of the flat nose projectile is the largest at 11.37% with an impact angle of 30°.