Dynamic compression experiments of fine grained Mg–7Gd–5Y–1.2Nd–0.5Zr alloy were measured by the split-Hopkinson pressure bar test at the strain rates in the range 1000–2000 s−1 and the temperature range 293–573 K along the transverse direction. The microstructure of the alloy was characterized by electron back-scattering diffraction and transmission electron microscopy. The results showed that the deformation hardening mechanisms dominated by pyramidal <c + a> slip and assisted by many mechanisms such as tension twinning, while the deformation softening mechanism just dominated by a partial dynamic recrystallization at the grain boundaries. During the entire deformation process at different temperatures, softening was found as the only accompanying mechanism of hardening. Even at 573 K, the fully recrystallized structure was not achieved, and the hardening mechanism was always dominant until they tend to balance. Based on these deformation mechanisms, especially the continuous attenuation mechanism of dynamic recrystallization softening associated with hardening, the Johnson–Cook model was modified, and a unified constitutive equation for deformation under high strain rate at different temperatures was constructed. The resulted obtained by this equation were in good agreement with the experimental results. •EBSD and TEM were used to analyze the deformation mechanism of Mg–7Gd–5Y–1.2Nd–0.5Zr alloy.•The deformation hardening mechanisms were dominated by pyramidal < c + a > slip and assisted by many mechanisms such as tension twinning, while the deformation softening mechanism was just dominated by dynamic recrystallization.•The Johnson–Cook model was modified, which was in good agreement with the measured results.