Terahertz technology can be utilized to analyze the internal structure and thickness of layered materials due to its advantages of overcoming the limitations of traditional nondestructive testing techniques and penetrating ordinary nonmetallic materials. The working principle of terahertz time domain spectral system(THz-TDS) on transmission mode is introduced, the wave propagation in multilayer structure is described by using Rouard equivalent interface theory, and the theoretic transmission model of terahertz wave in the three layers of the medium is derived on transmission mode. The transmission imaging is conducted on the prepared samples through the terahertz time-domain spectroscopy system. The results prove that the internal information such as the location and shape of polyvinyl chloride(PVC) sheet can be detected, which can provide theoretical and experimental basis for detecting the internal structure of multilayer material.