Mg2Si/AM60 composites were fabricated by in-situ synthesis and the effects of different stirring technological parameters on the microstructures of semi-solid magnesium matrix composites were investigated in this paper. The results indicated that there were Chinese script type Mg2Si particles in the microstructures by addition crystal Si to the magnesium alloys. The mechanical stirring was used to fabricate the semi-solid composites and the results showed that the higher the stirring temperature and speed are, the finer and more homogeneous the spherical grains would be. If the temperature is too high, the grains will be melted. With the increasing of stirring time, the grains become fine, well-distributed and round at first, then grow up.