This paper deals with the evolution law of microstructure and formation process of intracrystalline liquid pool of extruded AZ80 magnesium alloy after being treated through semi-solid isothermal treatment (SST) for different time. The result show that, with the semi-solid isothermal process, merger of grain coarsening occurs firstly, and then liquid phase appeares and expansion; with the increase of isothermal holding time, the α-Mg grains separated by liquid phase and tend to spheroid constantly, Ostwald ripening and grain coalescing will become the main mechanism with continue increase holding time. And the liquid pool inside the α-Mg grain is caused with the main mechanism of intracrystalline partial melting priority due to elements enrichment. And the partial zone with low melting cased with the solution elements enrichment melting priority is the main mechanism of intracrystalline liquid pool inside the α-Mg grain.