In this article the density functional theory and LDA are used to study the electronic and optical properties of newly discovered alloy superconductor MgCNi3.The calculated results exhibit that the conduction bands in this compound are derived by Ni 3d and C 2p states.The top valence states have essentially Ni 3d characters and the C 2p states occupy the region from 4.0eV to 7.0eV below Fermi energy.EF locates just at the step slope of sharply structured Ni 3dyz+zx and 3d3z2-r2 peaks near ＥＦ.The doping carriers from Mg atoms destroy the magnetic order.The calculated optical properties show that the optical conductivity in the energy range 0—12eV are contributed from the Ni 3d to Ni 4s and C 2p transitions.The density of states near Fermi energy N(EF) is evaluated to be 5.50 (states/eV·cell)which yielded a Sommerfeld constant γcal～4.45mJ/mol·K2.We conclude that the mechanism of superconductivity in MgCNi3 has the type of BCS with strong electron-phonon coupling.