Abstract: CBL/VDL logging is used to evaluate cementing quality under complex well conditions such as thick casing and thin cement sheath in deep and ultra-deep wells by using the amplitude and attenuation of the casing wave. However, the more research on the propagation mechanism of the casing mode waves is needed, especially in fine cementing explanation for deep cased wells. So the dispersion, attenuation and sensitivity curves of casing mode waves are calculated theoretically by establishing a casing well model with cylindrical multilayer medium. The results show that there are 3～4 order axial vibration modes in the range of logging frequencies, and the larger the outer diameter of the casing, the more vibration modes. Under the same casing diameter, the attenuation decreases and the excitation strength increases with the increasing of casing thickness, and the difference becomes more and more obvious with the increasing of cement impedance, and the amplitude changes little with free casing model. The sensitivity curve shows that the sensitivity of casing wave attenuation to shear wave velocity of cement is obviously higher than that of the compressional wave, which also explains that casing wave induced by monopole source leaks energy to cement mainly through shear coupling. In addition, the snapshot of the acoustic field shows that the casing wave will leak energy to the formation when propagating along the casing, so the change of cement annulus thickness and lithology will obviously change the casing wave amplitude. The theoretical calculation results lay a foundation for the precise interpretation of cementing quality evaluation in complex well conditions and the production of casing-wave amplitude correction charts.