Abstract: The generalized phonon density state of Y-Ba-Cu-O superconductor (T_c=90.5 K) is measured at 289 K and 87 K respectively, by using Be filter detectorneutron inelastic scattering spectrometer at Institute of Atomic Energy. At 87 K,the optical mode at 76 meV which is estimated owing to Cu-O-Cu breathing modeand a stronger acoustic mode below 6 meV are observed, and the low frequencymodes at 10 meV and 19 meV in 289 K seems to be disappeared, or to be softened.It is possible that the anomalous change of phonon density state below T_c isresponsible for the higher superconducting transition temperature of Y-Ba-Cu-O.