Good cement bond quality is significant for an oil well to prevent hydraulic communication between zones and to ensure layered exploitation. Conventional sonic logging techniques are limited to the detection region immediately behind the casing, so they can hardly be used to interpret the bonding quality between cement and rock formations. To develop a cementing evaluation method for the whole annulus, ultrasonic leaky flexural modes in radially multilayered media were investigated through finite-difference modeling. The guided waves were excited and received by pitch-catch transducers arranged in the borehole. According to the numerical results, two kinds of flexural modes were sensitive to cement debonding at different interfaces, respectively. For the primary flexural waves that arrived early and propagated all along the casing, the attenuation levels decreased due to the appearance of a fluid-filled channel between the casing and the cement sheath. The acoustic signals arriving later were the secondary flexural modes contributed by reflection echoes from the formation. They will be greatly strengthened once a debonding defect appears at the cement-formation boundary, and the arrival time of secondary flexural waves can also be applied to predict the thickness of the cement sheath. Consequently, it is possible to propose an integrated evaluation for both interfaces of the cement annulus by extracting properties of the primary and secondary flexural waves.