According to density functional first-principles calculations, we study the substitutional doping of Co atoms in core-shell silicon nanowires. By comparing the formation energies, we find that all the doping configurations obtained from shell-chain doping, core doping, and whole shell doping are stable, and core-shell doping silicon nanowire has the highest structural stability. All the doped configurations show metallic property, and the conductance channels increase with the increasing of doping concentration. Co-doped silicon nanowires show ferromagnetic, possessing magnetic moment. Bader charge analysis shows that charge is transferred from Si atoms to Co atoms in doped silicon nanowires. In transition metal Co atom, charge is transferred from 4s orbital to 3d and 4p orbital. The reducing of unpaired electron in 3d orbital and part of charge transferring from up-spin to down-spin in 4s, 3d and 4p orbital, makes magnetic moments in Co atom reduced.