Abstract: Regarding the calculation problem of the thickness limit of high-strength steel-concrete composite structures under the impact of slender thin-walled projectiles, this paper carried out tests on the impact of composite targets by slender thin-walled projectiles. Based on the test results, the protection mechanism of the composite structure and the failure modes of the projectile structure were analyzed. On the basis of the original thickness limit calculation model, considering the key factor of the projectile structure strength, a new thickness limit calculation model was then proposed, and the relevant parameters were empirically discussed. The research results show that: The protection mechanism of the high-strength steel-concrete composite structure lies in that the high-strength steel provides material strength, and the concrete back plate provides support stiffness, with the two complementing each other's advantages. Since the slender thin-walled projectiles are prone to compressive expansion and cracking failures during the impact process, the influence of the projectile structure strength on the impact effect must be considered in the calculation model. The design of the composite structure needs to take into account both the mechanical properties of the high-strength steel and the thickness limit of the composite structure. In addition, this paper points out that the calculation model has some shortcomings, such as the empirical nature of the parameters and the conservative nature of the calculation results. The model still needs to be revised in subsequent research. The research achievements of this paper can provide a theoretical basis for the design and application of high-strength steel-concrete composite structures in the field of protection engineering.