Abstract: Based on the simplified model of cased borehole,acoustic wave field in the borehole is numerically calculated when the bonding conditions of the first interface are different.The variation features of the two-dimension spectrum,excited spectrum and the waveform in time domain of acoustic wave field in borehole are investigated as the thickness of the fluid annulus at the first interface is varied.The numerical results show that two-dimension spectrum could reveal the changing law of each wave composition clearly.When the first interface is not boned while the second interface is well boned,the head wave is the casing wave,which is a guided wave traveling at a speed lower than the compres-sional wave in steel casing.Meanwhile,even there is a very tiny micro annulus between casing and cement annulus,the energy of casing wave will also increase sharply.The amplitude of the casing wave will increase with the increase of the thickness of the fluid annulus,however,when the thickness of the fluid annulus increases to a fixed value(such as 8mm),the increasing trend will be smooth.