<p>This study systematically explores the high-temperature deformation behavior of a dissolvable Mg-Gd-Ni-based magnesium alloy at 623-823 K and strain rate 0.1-10 s⁻¹, using the Gleeble-3800 thermal mechanical simulation system. The strain rate sensitivity index (<i>m</i>) and strain hardening index (<i>n</i>) were analyzed under various deformation conditions. A thermal processing map was constructed via the dynamic material model (DMM) framework and validated by macroscopic observations and microstructural characterization, delineating the hot working process window for the Mg-Gd-Ni-based alloy. The results show that the peak <i>m</i> values occur at elevated temperatures (760-790 K) and low strain rates (0.1 s⁻¹) across different strain levels, with a general decreasing trend in <i>m</i> as strain increases. The strain hardening index remains largely negative and decreases further at high strain rates, indicating a pronounced softening behavior. At these conditions, the microstructure exhibits distinct localized plastic flow bands. According to the thermal processing map, the optimal hot deformation parameters are identified as a deformation temperature of 760-790 K and a strain rate of 0.1 s⁻¹. In the regions corresponding to peak energy dissipation and outside the instability domains, dynamic recrystallization progressively fragments and replaces the original grains, resulting in a refined and uniform microstructure.</p>