In this paper. Tang's tensor operator method is applied to a fitting calculation of crystal-field energy levels of the rare earth ions. The splitting chain of energy levels of LiYF4: Pr3+,crystal-field parameters and approximate wave functions cor-reoponding to the crystal-field group chain R3?Td?D2d?S4 are ohtoined. These results can he used to analyse various prohlems, such as space charge distribution, spectral line intensity and other optical, magnetic and thermodynamic properties in cases with different approximate symmetry.