Geometrical structures, stability, electronic properties, vibrational spectrum and polarizations of BeSi_n(n=1-12) clusters are investigated with density functional theory (DFT) at B3LYP/6-311G level. It indicates that there are many isomers with extremely close energies near the ground state. The most stable structures of BeSi_n clusters favor three-dimensional structures as n≥4. At n=1 spin multiplicity of the ground state structure of BeSi_n cluster is triplet while it is singlet as n≥2. Electronic properties of host clusters change obviously due to encapsulation of Be atom. Doping of Be atom reduce chemical stability of pure Si clusters, n=3, 5, 7 and 9 are magic numbers.