Spinel ferrite is one of the very important magnetic materials, having the unique physical properties, chemical properties, magnetic properties, and electronic properties. CoFe2O4 is widely used due to their good electromagnetic properties. We have studied the electronic structure and magnetic properties of CoRE0.125Fe1.875O4 (RE = Nd, Eu, Gd)by first-principles plane-wave pseudopotential method based on density functional theory (DFT), combined with the generalized gradient approximation (GGA + U) in this paper. Results show that the lattice constants of the compunds CoFe1.875RE0.125O4 (RE=Nd, Eu and Gd) will decrease due to the decreasing ionic radius of RE as the atomic number increases. Their magnetic properties depend on the unpaired 4f electrons of RE3+ ions, and the net magnetic moment of CoFe2O4 will increase with Eu and Gd doping, mainly because there are more unpaired 4f electrons in Eu3+ and Gd3+. Thus the doping of Eu3+ and Gd3+ may have a greater impact on the magnetic properties of cobalt ferrite. The contribution from the doping of Nd is not remarkable on the magnetic properties, since the Nd3 + ion, having a larger ionic radius, could distort the crystal structure of CoFe2O4.