The study aims to solve the problem of calculating the thickness limit of high-strength steel-concrete composite structures under the impact of slender thin-walled projectiles, a key consideration for protective engineering design. A series of impact tests on composite targets were carried out. These targets were composed of different high-strength steel plates and concrete backplates. Slender thin-walled projectiles were launched with a gas gun at controlled velocities, and the impact process were captured by high-speed cameras. The resulting damage to the structures and the failure modes of the projectiles were analyzed using both non-destructive and destructive testing methods. Based on test results, the protective mechanism of the composite structures and the failure modes of projectiles were analyzed. An improved thickness limit calculation model was then developed. Unlike the original model, this new model incorporated the structural strength of slender thin-walled projectiles, considering their wall thickness, material yield strength, and geometric dimensions, and was established based on force equilibrium and energy conservation principles. The results show that the high-strength steel in the composite structures provides material strength to resist penetration, while the concrete backplate offers support stiffness. As slender thin-walled projectiles are prone to compression and expansion cracking during impact, their structural strength must be factored into the calculation model. Moreover, the design of composite structures should consider both the mechanical properties of high-strength steel and the thickness limit. In conclusion, though the proposed model offers a new theoretical approach, it has limitations such as empirical parameters and conservative results. Further research is necessary to refine and enhance the model. The study's findings provide a theoretical basis for the design and application of high-strength steel-concrete composite structures in protective engineering.