The microstructure and impact toughness of in-situ Mg2Si/Al-Si composites were studied in the different content of Sb. The results show that Sb can improve the microstructure and impact toughness of Mg2Si/Al-Si composites. When the content of Sb is 0.4%, the morphology of primary Mg2Si changes from dendrites to fine particles, the average size of Mg2Si particles is refined from 52μm to 25μm, and the impact toughness of the composites increases from 6.3572J/cm2 to 11.4394J/cm2. The improvement of impact toughness can be attributed to the fine-grain strengthening. However, excessive Sb is disadvantageous to the modification of the composites.