Abstract: Low-impedance cementing quality evaluation is an urgent subject to be solved in oil and gas production. The lowest order antisymmetric mode of leaky Lamb waves (hereinafter referred to as Lamb flexural mode) have been used for cement evaluation because it is sensitive to the acoustic parameters of layered media (especially the low- impedance cement) and cementation condition. An investigation into the relationships among flexural mode dispersion and attenuation, cement acoustic properties, and bonding conditions between casing and cement is conducted using multi-layered media model. The full waveforms launched and received by finite and directional emitting and receiving transducers are simulated then. The result shows that the flexural mode dispersion curve is broken into two branches when slow cement exists behind casing; the flexural mode attenuation maximizes when slow cement exists behind casing and it is sensitive to channel fluid thickness between casing and cement. We can distinguish the type of layered media behind casing, bonding conditions between casing and cement and also channel fluid thickness in the presence of free pipe condition effectively by observing the flexural mode attenuation.