Based on the feature of molecular crystal, we apply the theorem that the foctor group of the space group is isomorphic to the point group and combine with the method of structural analysis, to classify the modes of internal vibrations of RbC8H5O4 directly according to the compatibility relations of the irreducible representations. For spectra of the fundamental frequency coupling and Fermi resonance in vibration spectra, we have to make determination according to the space relative position of each functional group and symmetrization of each fundamental direct product representation. Raman scattering spectra of three kind of isomor-phous crystal (i.e, RAP, TAP and KAP) have been obtained and compared with each other. The modes of external vibrations in crystal lattice was then isolated from the overall spectra.