Abstract: This study investigates the resistance mechanism of steel bars in reinforced concrete structures against local projectile penetration using a combination of experimental tests and numerical simulations. Comparative penetration tests are conducted for projectiles impacting steel bars and passing through steel bar openings. The results show that the penetration depth is significantly smaller when the projectile directly impacts the steel bars, which directly verifies the resistance effect of steel bars. A high-precision numerical simulation method is further established and validated against the test data, based on which the resistance mechanism of steel bars to projectile penetration is analyzed. The main conclusions are drawn as follows: (1) The resistance of steel bars against projectile penetration exhibits an obvious contact-dependent characteristic. When no direct contact occurs, steel bars with different diameters show negligible difference in improving the anti-penetration performance of structures. In contrast, prominent resistance is observed when the projectile impacts steel bars or their intersections, and the resistance effect increases with the steel bar diameter. (2) Statistical analysis on the energy evolution of the whole system indicates that the impact on steel bar intersections produces high strain energy in both the steel bars and the deformed projectile, which alleviates the structural damage degree. (3) The resistance performance of steel bars is significantly affected by reinforcement layout. When the spacing of steel bars is considerably larger than the projectile size, the contribution of steel bars to the structural anti-penetration capacity is extremely limited. This study clarifies the contact resistance mechanism, energy dissipation law, and the influence of reinforcement layout on the anti-penetration performance of reinforced concrete structures, providing a theoretical reference for the optimal design of anti-penetration reinforced concrete structures.