The microstructural evolution of AZ61 semi-solid magnesium alloy during semi-solid remelting process was studied in this paper. The semi-solid billet was fabricated by strain-induced melt activation (SIMA) method. The results showed that the initial semi-solid grains melt mainly through coalescence. With the prolongation of isothermal holding time, the grains grew up and spheroidized, in which the mean diameter of grain and liquid volume fraction increased. In the meantime, the higher the holding temperature, the faster the grain grew and spheroidized. The suitable reheating temperature of AZ61 semi-solid magnesium alloy was 592. The samples were susceptible to serious deformation beyond 597.