Abstract: Cementing quality evaluation is critical for ensuring the integrity of oil and gas wellbores, achieving effective interlayer isolation, and supporting the safe development of oil and gas resources. However, conventional evaluation methods have limited capability to quantitatively characterize localized irregular cement loss, microannuli, and heterogeneous annular media. In this study, a phased-array source loading scheme is used to simulate the directional acoustic beam in ultrasonic flexural-wave logging. Based on the two-dimensional elastic wave equation, an acoustic-pressure waveform residual objective function is constructed, and the adjoint-state method and L-BFGS algorithm are combined to simultaneously invert the elastic-parameter models of annular media behind casing. On the basis of two-dimensional numerical experiments, the effects of casing eccentricity, wellbore-fluid parameter errors, undulating cement–formation interfaces, localized small-scale cement loss, and cement microannuli on full-waveform inversion are analyzed. The results show that the proposed method can make full use of complete wavefield information, including A0-mode waves, TIE reflected waves, and scattered waves, to reconstruct the elastic-parameter models of high-impedance cement annular media behind casing. This provides theoretical and technical support for the high-resolution and accurate characterization of heterogeneous cement annular media.