<p>When using numerical simulation to study the destruction of concrete under the action of projectile penetration, the Holmquist-Johnson-Cook (HJC) constitutive model is one of the most commonly used in concrete, and the reliability of its parameters is a crucial factor affecting the accuracy of numerical simulation results. This paper first prepared C100 grade ultra-high-strength concrete (UHSC) and characterized its fundamental mechanical properties. Based on these properties, the parameters of the yield surface equation, damage equation, and state equation in the corresponding HJC constitutive model for concrete were accordingly updated, resulting in a set of HJC constitutive model parameters applicable to UHSC. Second, using the LS-DYNA software, a numerical model of a reinforced UHSC military protective slab (RUHSCS) was constructed to investigate its mechanical responses under the projectile penetration. The simulated depth of penetration (DOP) using the updated parameters showed excellent agreement with experimental data, validating the model’s reliability. Subsequently, a parametric study on rebar configuration revealed that reducing rebar grid spacing is the most effective measure in decreasing DOP, followed by reducing layer spacing and increasing rebar diameter. Furthermore, it was demonstrated that for a given volumetric rebar ratio, employing smaller-diameter rebar with denser grid and layer spacing constitutes the most efficient reinforcement strategy for enhancing the ballistic anti-penetration of RUHSCS.</p>