The formation of the microstructure in the rheo-diecastings were studied by the experiment of rheo-diecasting with semi-solidA380 Aluminum alloy slurry. The results show that the grain diameter and shape factor of primary α-Al grains decreased andincreased along the rheo-diecasting direction, respectively, making the primary α-Al grains gradually distributing uniformly. The combination of injection pressure and narrow gate of the runner and ingate was the main reason for the evolution of theprimary α-Al grains, moreover, the collision and friction among primary α-Al grains during the flling were helpful for therefnement and spheroidization of the primary α-Al grains. The residual liquid solidifed into the secondary solidifcationmicrostructure accompanied by the evolution of the primary α-Al grains. The accumulation of the fragments of the primaryα-Al grains, the secondary α2-Al nuclei or grains which formed in previous positions, the efective nucleation rate of theresidual liquid, and the collision and fraction among the secondary α2-Al grains and between the secondary α2-Al grainand the primary α-Al grains made the secondary solidifcation microstructure of the residual liquid present multiplex andcomplicate characteristics.