Based on the fact of the existance of a small amount itinerant electrons, a new theoretical model giving rise to magneto-crystalline anisotropy in compound Nd2Fe14B is presented in this paper. It is proposed that a combined effect on Nd-ion coming from both the crystal field of ligands and the electric field of itinerant electrons results in the anisotropy in the compound. An approach of calculating the interaction between itinerant electrons and central Nd-ion is given. The magneto-crystalline anisotropy of Nd-ion and its temperature dependences are calculated based on the model. The results are in good agreement with the experiments, and also give explanation to the spin reorientation phenomenon in the compound.