Abstract: To investigate the dynamic mechanical properties of Ultra-High Performance Concrete (UHPC) under the coupling effects of high temperature and blast impact, a high-temperature split hopkinson pressure bar (SHPB) testing system was employed to conduct unconfined compression tests on C140 UHPC. The tests covered a temperature range of 25–600 °C and a strain rate range of 90–200 s-1. Systematic analyses were performed on the material’s strength, strain capacity, toughness, stress-strain relationship, and failure modes under the coupled high-temperature and impact conditions, aiming to elucidate the influence laws of temperature and strain rate effects on its dynamic mechanical performance. Furthermore, the yield surface of the Holmquist-Johnson-Concrete (HJC) constitutive model was modified to account for temperature effects. The results indicate that UHPC exhibits a pronounced strain rate hardening effect under high-temperature dynamic compression; However, high temperatures simultaneously deteriorates its mechanical properties. The evolutionary behavior of the material’s strain capacity and toughness arises from the synergistic effects of temperature and strain rate. At a given temperature, an increase in strain rate intensifies the degree of UHPC failure. When the temperature exceeds 400 °C, matrix deterioration and steel fiber oxidation of UHPC lead to overall brittle failure characteristics; nevertheless, the local core region remains intact and retains significant residual load-bearing capacity. The modified HJC yield surface is applicable to studying the dynamic mechanical properties of this type of material under the coupling effects of high temperature and impact. Ultimately, these insights—encompassing the coupled effects of temperature/strain rate and the modified HJC yield surface—offer a robust theoretical foundation and reliable data support for the safety design and performance assessment of military-civilian protective structures.