High pressure solidification rules of Al-Mg alloy needs to be discussed further for its wide range of application. Microstructures and phases of Al-25wt% Mg alloy solidified at 4 GPa were studied by optical microscope,X-ray diffractmeter,energy dispersive X-ray spectroscopy and transmission electron microscopy( TEM). The microstructure evolution mechanism of Al-25Mg alloy under high pressure was analyzed. The result shows that the alloy consists of α-Al phase and Al 3 Mg 2 phase under normal pressure. However,only Al 12 Mg 17 phase forms without Al 3 Mg 2 phase at 4 GPa. In addition,Mg concentration in α-Al phase increases and that of the lattice constant also increases. The α-Al dendrite presents the broken arms under normal pressure, after high pressure solidification,the morphology of the dendrite tends to integrate and the size of the dendrite arms decreases Microstructures and phases of Al-25 wt% Mg alloy solidified at 4 GPa were studied by optical microscope, X-ray diffractometer, energy dispersive X- The result shows that the alloy consists of α-Al phase and Al 3 Mg 2 phase under normal pressure. However, only Al 12 Mg 17 phase forms without Al 3 Mg 2 phase at 4 GPa. In addition, Mg concentration in α-Al phase increases and that of the lattice constant also increases. The α-Al dendrite presents the broken arms under normal pressure, after high pressure solidification, the morphology of the dendrite tends to integrate and the size of the dendrite arms decreases