Microstructure, characteristic and metamorphism of Mg2(Si,Sn) compound phase are studied in as-cast Mg-Sn-Si alloy. The results indicate that Sn atom can replace the partial Si of Mg2Si then form Mg2(Si,Sn) compound phase, and structure of the ternary compound is the same as Mg2Si and Mg2Sn, all belong to face-centered cubic. The element content of Mg2(Si,Sn) phase is not invariable, the Mg2(Si,Sn) phase with high Si content is in Si enrichment region, while it with low Si content is in Si poverty region. The property of Mg2(Si,Sn) phase with high Si content is close to Mg2Si phase, the property of the phase with high Sn content is close to Mg2Sn phase. In experiments, the nano-hardness, elastic modulus, vickers hardness and other physical properties of Mg2(Si,Sn) phase are between Mg2Si phase and Mg2Sn phase, in the metamorphism process of Chinese-script-like Mg2Si phase, Mg2(Si,Sn) compound phase plays a role as a bridge.