Revealing the interactions of sound waves with both SiC particles and internal defects is crucial for facilitating the detectability of internal defect features in SiCp/Al by using ultrasonic testing (UT). In the present work, we demonstrate the feasibility of UT of internal flat-bottom holes with diameters ranging from 0.2 mm to 2 mm in SiCp/Al composites through the combination of finite element (FE) simulations and experiments. Specially, a 2D FE model of UT of SiCp/Al with consistent geometrical features of SiC particles with experimental one is established, the accuracy of which is validated by theoretical and experimental characterizations of P-wave velocity and ultrasonic attenuation coefficient of SiCp/Al. Subsequently, the propagation behavior of sound waves in the SiCp/Al specimen with pre-existing defects under UT, in particular the impact of defect boundary on the scattering behavior of sound waves, is revealed in detail by FE simulations and also validated by corresponding experiments. Furthermore, the UT limit of detectable size of the internal defects is revealed jointly by FE simulations and experiments, based on which a correlation map between defect size and echo signal amplitude is established. Current study provides theoretical and practical guidance for the UT of internal defects in SiCp/Al composites.