Based on the fact of the existence of a small amount itinerant electrons, a new theoretical model giving rise to magneto-crystalline anisotropy in compound Nd_2Fe_14_B is presented in this paper. It is proposed that a combined effect on Nd一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 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.