Abstract: In compliance with the crystal structure proposed by Binnie, the principal vibrations of Pb²MoO⁵ crystal is analysed. The symmetry types of all characteristic modes are calculated and then, the symmetry types of the internal and external vibration modes of the MoO₄^2- and other ions are further calculated by means of factor group analysis. The experimental results are compared with the group-theoretical calculation, and with the Raman spectral lines of PbMoO⁴ and other scheelite crystals. The distribution of the spectral lines and their symmetry show explicitly the effect of internal vibration of MoO₄^2- in Pb²MoO⁵ crystal.