Raman spectra of BiB_3O_6 crystal(BIBO) was studied.It is an approximate treatment to divide the vibrational modes of the BIBO crystal into internal and external modes,the total number of external motions of BIBO are 2A+5B,the internal motions of the BIBO are 11A+10B.By using the method of molecular site group analysis,we attributed tentatively the internal modes BIBO to the vibrational modes of the BO_3 triangle and BO_4 tetrahedron,the BO_4 tetrahedron and BO_3 triangle share oxygen atoms.The external vibrations originated from the translational motions of Bi~(3+) and triborate anion group plus the librational motion of triborate anion group.