Abstract: When using CBL/VDL logging to evaluate cementing quality under complex well conditions such as thick casing and thin cement sheath in deep and ultra-deep wells, the measured amplitude or attenuation of the casing wave is used to determine the cementation condition of cement sheath. However, the research on the propagation mechanism of the casing mode waves is insufficient, and there is a lack of theoretical support for precise cementing quality evaluation. Therefore, a cased well model with cylindrical multi-layer medium was established, and the phase velocity, attenuation, and sensitivity curves of the casing mode waves were calculated. The effects of non-cementing factors, casing diameter, and cement sheath size on casing wave attenuation were studied. The results show that there are 3~4 order axial vibration modes of the casing wave within the range of CBL/VDL logging frequencies. The larger outer diameter of the casing, the more vibration modes there will be. When the casing is cemented, the amplitude of the casing wave increases obviously as the casing thickness increases, while the amplitude of casing waves basically remains unchanged for free-standing casing. Sensitivity curves show that the attenuation of the casing wave is more sensitive to cement shear wave velocity than to the compressional wave velocity, suggesting that casing wave induced by monopole source leaks energy to cement sheath mainly through shear coupling effect. In addition, the snapshots of shear wave and compressional wave fields show that the casing wave also leaks energy to the formation when propagating along the casing, so the change in cement sheath thickness and lithology will also affect the amplitude of the casing wave. The research results provide a theoretical basis for the precise evaluation of cementing quality nd the creation of calibration chart for the casing wave amplitude in complex well conditions.