In this paper, the diffraction efficiency as a function of grating writing angle in LN∶Fe, Hf crystals was studied, and the relationship curve was fitted and analyzed. It is found that the bulk photovoltaic coefficient κ increases significantly when the Hf-doping concentration exceeds the threshold. The dramatic increase of the coefficient κ to the fact that the Hf-doping eliminates the intrinsic defects in the crystal and the perfection of the lattice environment enhances the photovoltaic coefficient k of the Fe ions remaining at Li sites. The experimental results also show that the concentration of photorefractive centers is about 14.5 ppm (1 ppm=10-6) in LN∶Fe, Hf crystals, that is, 4.8% of the Fe ions still “remain” in the Li site even though the Hf-doping concentration already exceeds the threshold, and that these Fe ions can cause sufficiently strong photorefractive effect and become the dominant photorefractive centers in the LN crystal. In addition, the possibility of Fe ion occupation in LN lattice from the perspective of impurity-hydroxyl defect complex was discussed.