We use the first-principles method to calculate the electronic band structures of MgCNi3. The calculated results show that the hybridization between C 2p and Ni 3d electron orbits leads to the planar characteristic of Ni 3d band. The Fermi level is located on the right slope of the van Hove singularity (vHs) peak. The high density of states (DOS) at the Fermi level with vHs and large magnetic fluctuations near the ferromagnetic phase transition point are the important factors for the superconductivity of MgCNi3. We study the variance of superconductivity and magnetism for three types of substitute doping in MgCNi3, and find that the electron doping moves the Fermi level to the lower DOS side, and transforms MgCNi3 to paramagneticm phase without superconductivity. The inter-metal compounds with the same valence electrons doping change the shape of Fermi surface, resulting from the hybridization between atoms. The decrease of the DOS decreases their superconductivity. The hole doping enhances the DOS at the Fermi level on the peak of vHs, but the large magnetic exchange interactions induce the ferromagnetic order, and the superconductivity disappears abruptly.