Abstract: Numerical simulation based on finite element method was carried out for the air-coupled ultrasonic testing of C/SiC composites with high porosity. The elastic stiffness matrix was calculated based on mechanical and acoustical analysis. Finite element models with random void morphology were established, of which the porosity was 5%, 10%, and 15%, respectively. The characteristics of ultrasonic propagation and the response to typical defects were studied in the transmission testing profile. Results indicate that the longitudinal wave velocity in the direction vertical to laminate is about 2830 m/s, and five independent elastic constants of transverse isotropy are 158.149, 88.589, 34.141, 15.288 and 13.793 GPa. The voids are strip-like, and the ratio of width to wavelength is between 0.05~0.22. It is found that the ultrasonic attenuation gradually increases with the increasing porosity, which is mainly in Rayleigh scattering regime. Meanwhile, the directivity pattern of ultrasonic field under some conditions is also changed due to the high porosity and complex morphology. As the length of delamination increases from 0 to 25 mm, the attenuation of received signal increases, and the maximum is about 33.9 dB compared with that without delamination. It is of similarity for the evolution of the ultrasonic field with the increasing thickness of laminate, mainly owing to the combined effects of the delamination and voids. The simulation results show a good consistency with experiments and provide support for high-quality non-destructive testing on C/SiC composites.