Based on the first-principles method of density functional theory (DFT), the segregation behavior of Si, Ni, Mn, Cr, and Mo at the NbC/fcc-Fe interface was studied, and the influence of B on the segregation behavior of alloying elements was analyzed. The results show that Cr and Mo can stably exist in the interface and NbC. Mo tends to segregate in the interface and NbC; Ni and Mn have a slight tendency to interface segregation. Mo is easy to segregate into NbC to form composite carbides. When the Mo/Nb content ratio is less than 2/3, the (Nb, Mo)C composite carbide is more stable and the binding energy is greater, which should be related to the strong electronic interaction between Mo and C and between Mo and Fe. When B is doped to the interface, the tendency of Mo and Cr to segregate to the interface is suppressed, expecially the segregation of Mo at the interface, thereby improving the corrosion resistance of the material. In addition, B can make Ni and Mn tend to be uniformly distributed in the matrix.