The flow characteristics and deformation mechanism of Al−Mg−Si alloy were studied at various temperatures (77−298 K) and strain rates (900−7000 s⁻¹) using the Hopkinson pressure bar method, electron backscattered diffraction (EBSD), and transmission electron microscopy (TEM). The results showed that increasing the strain rate and decreasing the deformation temperature significantly enhanced the work hardening ability of Al−Mg−Si alloy, thereby markedly improving the plasticity. A dislocation density-based constitutive model for the Al−Mg−Si alloy was established, incorporating dislocation accumulation and dynamic recovery mechanisms, which accurately described the flow behaviors under different conditions. Microstructural observation revealed that the combination of cryogenic temperature and high strain rate significantly suppressed dislocation cross-slip, which led to the formation of numerous slip bands. As strain accumulated, these slip bands interacted and facilitated recrystallization, thereby obviously accelerating the grain refinement process.