Abstract: To analyze the effect of strength mismatch on the ballistic performance of aluminum welded joint, a model of aluminum alloy plate penetrated by 7.62mm bullet was created, and the Johnson-Cook constitutive equation was adopted. Results show that the penetration direction will change due to the heterogeneous deformation as the projectile penetrates the aluminum weld joint, and the magnitude of angle change of penetration direction relates to the penetration position, impact velocity, projectile geometry, the rate of strength mismatch and target thickness. When the projectile only penetrates the region of weld seam at a small impact velocity, material’s ballistic performance may be worse than that of pure weld seam material due to material deformation localization. There are some regions whose ballistic performances are better than that of aluminum base due to the change of penetration direction. It is useful for the ballistic performance analysis of heterogeneous materials.