Abstract: This study investigates the dynamic and quasi-static mechanical behaviors of a single-phase body-centered cubic (BCC) WNbMoTaV refractory high-entropy alloy (RHEA) fabricated by vacuum arc melting, over a temperature range of 77.15–1373.15 K and strain rates of 0.001–7300 s⁻¹. The microstructural evolution mechanisms under thermo-mechanical coupling were explored. Based on experimental data, the Johnson–Cook (J–C) constitutive model was fitted and analyzed. Results indicate significant deviations in the strain-rate hardening stage, with pronounced temperature dependence of mechanical responses: the flow stress at 77.15 K was only 68.5% of that at 1373.15 K under certain conditions. Therefore, a modified J–C model was proposed by introducing regression coefficients and a coupled temperature–strain rate correction term for both strain-rate hardening and thermal softening. Finite element numerical simulations verified that the error range of the modified model lies between 8.0% and 12.5%. Furthermore, microstructural characterization via scanning electron microscopy (SEM), electron backscatter diffraction (EBSD), and high-resolution transmission electron microscopy (HRTEM) revealed that on cleavage surfaces of quasi-static tensile fracture, tough tear ridges and river patterns exhibited distinct distributions, along with varying degrees of grain refinement. Fracture and crack propagation modes showed notable evolution. Under combined temperature and impact loading, the high proportion of high-angle grain boundaries (HAGBs) was identified as the core factor for the wide-temperature stability of the WMoTaNbV RHEA. At low temperatures, sub-grain refinement and dislocation walls enhanced dynamic strength, whereas at high temperatures, reduced lattice distortion and dynamic recrystallization jointly promoted the formation of HAGBs.