Stable structures and Raman spectra of guanine have been investigated by density functional theory (DFT). Focusing on solvent effect and hydrogen bonding interaction, we have calculated the two keto-amino tautomers G17K and G19K as well as their guanine-water complexes and tetramers. The results show G17K is more stable than G19K in the gas phase, whereas in polar solvents G19K dominates. The vibrational fundamentals of G17K have been reassigned based on normal-mode analysis, since the previous assignment was limited to the G19K only. In the Raman spectra, the modes of the ring breathing vibration and those in the fingerprint region (from 1000 to 1600 cm(-1)) affected by the solvent effect and the hydrogen bonding interaction dramatically. The band at 1163 cm(-1) of G17K in gas has a large blue shift when water molecule forms hydrogen bonds with N-7-H-16 and C-6=O-13 sites. The blue shift can be explained by the influence of hydrogen bonding interaction along with shortening the N-1-C-6 bond distance. In addition, the dominant existing tautomer in polycrystalline and powder guanine is proposed to be G17K, whose calculated vibrational frequencies agree with the experimental Raman spectra reported before.