Abstract: In order to investigate the armor-piercing penetration mechanism and thickness limit calculation method of high-strength steel-reinforced concrete composite protective structure under the high-speed impact of projectile, and to provide the basis and reference for the design of this kind of composite protective structure, this paper determines the armor-piercing pattern of high-strength steel-reinforced concrete composite structure through projectile penetration model test of composite target, and carries out the decoupling analysis of the force of the front-covered high-strength steel plate and the reinforce concrete backboard, and establishes a theoretical model of the critical armor penetration of high-strength steel-reinforced concrete composite protective structure. Based on the impact dynamics theory, a total of three types of thickness limit control equations are established for high-strength steel without backboard support, high-strength steel with infinite thickness backboard support, and reinforced concrete backboard, and an overall thickness limit calculation method for high-strength steel-reinforced concrete composite protective structure is proposed in combination with the thickness limit. At the same time, the validation test of projectile penetration into the high-strength steel-C40 reinforced concrete composite target was designed and carried out, and the test results proved the reliability of the calculation method and revealed the anti-penetration mechanism of the high-strength steel-reinforced concrete composite structure. Finally, the applicability and error of the calculation method are discussed according to the assumptions of the theoretical model. The results of the research show that the theoretical calculation results are generally consistent with the results of experimental verification, proving that the theoretical model can effectively predict the thickness limit of composite structures. Since the mass loss of the projectile and the dynamic strain rate effect of the material are not taken into account, the theoretically calculated thickness limit results are higher than the experimental validation data, which is on the safe side for the design of engineering protection.