Abstract: Steel-concrete (SC) composite structures are widely applied in nuclear engineering protective facilities, whose anti-impact and anti-penetration performance is critical to structural safety under extreme loading conditions. To comprehensively clarify the dynamic mechanical behaviors and key parametric influence mechanism of typical nuclear SC structures under projectile impact, this paper adopts a hybrid research method combining experimental testing and numerical simulation. A 1/5 scaled SC structural model was designed and manufactured, and impact penetration tests were conducted to systematically explore the influence of impact velocity as well as the spacing of internal stiffeners and studs on structural deformation and anti-impact performance. Based on the test data, a refined numerical model was established, where reasonable material constitutive models and accurate interface contact parameters were calibrated, achieving good consistency between simulation and experiment. The test results show that the projectile penetration depth increases linearly with growing impact velocity before obvious deformation appears on the target rear steel plate. The front steel plate bulge deformation presents no obvious correlation with impact velocity, while the rear bulge deformation increases with impact velocity before structural penetration. Once the target is completely penetrated, continuous velocity improvement barely changes the rear deformation. Dense arrangement of stiffeners and studs can effectively constrain the deformation range of both front and rear steel plates. Numerical verification shows that the simulated failure modes, penetration depth and bulge deformation dimensions agree well with test results, with relative errors within 15% and maximum bulge height error less than 9 mm, proving the validity of the numerical method. Further mechanism analysis indicates that dense connection systems delay steel plate deformation but slightly reduce its energy dissipation efficiency, leading to marginally larger penetration depth and residual velocity. Nevertheless, concrete dominates the anti-penetration capacity of SC structures, while the steel plate contributes less than 10% of total energy dissipation, making the influence of connection layout relatively limited. In addition, prototype structure comparison reveals that the prototype suffers more severe damage than the scaled model under identical impact conditions. Appropriately increasing steel plate thickness can enhance structural energy absorption and mitigate damage under critical failure states, yet steel plate thickness only imposes a minor effect on the overall anti-penetration performance of SC structures.