The microstructure of the 7075 aluminum alloy, which was extruded before the preparation of a semi-solid isothermal remelting billet, was studied in this paper along with the effects of isothermal temperature and holding time. The grain coarse-grain law was predicted using the grain-growth model in the JMAK module of DEFORM-3D software. The findings demonstrate that the grain size gradually rises, the grain tends to be rounded, and the liquid content of the grain border increases with increasing isothermal temperature and holding time. During the high solid fraction interval, the grains underwent recrystallization, merged and grown, and liquid phase increased. In the low solid fraction interval, non-recrystallized grains are rarely visible, and Ostwald ripening is the primary mechanism causing the grains to grow quickly. In the semi-solid isothermal remelting grain growth formula, n=3 is a more accurate value. The rate of grain growth is calculated at 55.1 μm3/s, 295.3 μm3/s and 817.9 μm3/s as isothermal temperature rises. Using the energy relation and grain growth formula, the activation energy of the grain boundary migration of the 7075 aluminum alloy was calculated to be 118.6 kJ/mol, with a grain growth rule in the 550-620 °C range predicted. The actual measured value and the expected value are relatively close.