The possible geometrical structures of Mg2Sin(n=1~9) clusters are performed structural optimization by using the density-functional theory (DFT) at the B3LYP/6-311G (d) level. For the lowest energy structures, the stabilities and spectrum properties are investigated. The calculated results indicate that the most stable structures of Mg2Sin clusters favor the three-dimensional structures when n≥3. The impurity magnesium atoms in the Mg2Sin clusters can reduce the chemical stability of silicon cluster with small size. Mg2Si4 and Mg2Si6 are the magic numbers. The number of the infrared vibrational peak for Mg2Si4 cluster is only one, but the Raman vibrational peaks are much more. The Mg2Si4 cluster exhibits strong Raman activity at higher frequency band. On the contrary, Mg2Si6 cluster has more Raman vibrational peaks and one infrared vibrational peak, it exhibits strong infrared activity at the whole frequency band.