Variations of electronic properties of bulk α-Fe(C) due to the addition of Cr, Mo, Ni are studied using the first principles method, which is based on the density functional theory (DFT). Lattice distortion, cohesive energy, density of states, Mulliken charge population, overlap population and charge density differences are calculated by the generalized gradient approximation (GGA)-PW91, and the occupancy, bonding characters and alloying effect of α-Fe(C) after mixing with alloy atoms are explained. Results show that Cr occupies perferentially the corner of α-Fe(C) unit cell, but Mo and Ni occupy the body-center position of the cell perferentially. The mixing with Cr produces the biggest cohesive energe, and that with Ni and Mo in turn produces lower cohensive energy. There coexist the metallic bond, covalent bond, and weaker ionic bond in the unit cell. The bonding orbitals are created by Cr3d with Fe3d, Mo4d with Fe3d, and Ni3d with Fe3d and C2p; the bonding force between Cr and other atoms in the bulk is powerful, and the stability of cell is good, so it helps improve the mechanical strength of steels. The bonding force between Ni and other atoms in the bulk is weaker, but it can also maintain the stability of the cell. Though the bonding force between Mo and other atoms in the bulk is powerful, but the antibonding is so strong, that the statility of the cell reduced greatly. This is harmful to the mechanical strength of steels.