Optical microscope (OM), energy dispersive X-ray (EDX) analysis, differential scanning calorimetry (DSC), X-ray diffraction (XRD) and transmission electron microscope (TEM) were applied to investigate the solidification microstructures and phases of Al-9.6 wt.% Mg alloy which solidified under 4 GPa high pressure with the melting temperature 1,153 K. Fine dendritic microstructures were obtained, and the second dendritic arm spacing reduced. Area fraction of the primary alpha-Al phase increased and that of the second phase decreased. In addition, the solid solubility of Mg in alpha-Al phase increased. The lattice constant of alpha-Al phase increased. Specially, the new double phase regions (alpha-Al' + Al (x) Mg (y) ) formed besides a small amount of Al3Mg2 phases under high pressure. The Al (x) Mg (y) phase presented a mean size of about 20 nm, and had the hexagonal structure with the lattice constant of a = 0.288 nm, c = 0.8165 nm probably. Wherein the lattice constant of alpha-Al' phase differed from that of alpha-Al phase greatly. Moreover, evolution mechanism of microstructures and phases under 4 GPa high pressure was discussed.