Abstract: Thermal wave imaging (TWI) technology was used to examine the density change of the 3D carbon fiber needled preform with density gradient, tracking the transmissibility of the internal defect before and after the chemical vapor infiltration (CVI) silicon carbide (SiC) process of the preform. The reliability of the results was validated by X-ray radiography and industrial computed tomography (CT). The results show that the cavities in the preform are filled well after the CVI process, in which, however, some new cavities or holes generated from SiC matrix. It indicates that the cavity defects transmissibility can be detected and tracked by TWI technology successfully and the density gradient of the 3D needled perform changes reversely after the CVI SiC process.