In industrial applications， only one side of the composite media can be detected in most cases， so it is necessary to study the terahertz nondestructive testing method to detect the internal parameters of the desired media based on the reflection mode. The transmission model of terahertz waves in the medium of a three-layer structure is derived under the reflection mode. The thickness of the middle layer together and its refractive index are estimated by a genetic algorithm to attain the specific information about the middle hidden layer. At the same time， samples with the three-layer structure consisting of the hidden layer with the thickness of 200 µm are prepared and subjected to a terahertz time-domain spectroscopic system. In addition， a theoretical model is designed to compare the achieved measurements. Thereafter， a genetic algorithm is designed to estimate the thickness of the hidden layer and the respective refractive index to verify the effectiveness of the proposed method. Compared the estimated value of thickness with the measurement result of the thickness gauge， the error is kept within 4%. Compared with the actual value， the error range of the estimate refractive index fluctuates greatly， the average error is about 6%. Finally， the error sources is analysed， which provides the theoretical and experimental basis for the iternal defects of the multilayer composite material and the dielectric parameter of the intermediate layer material. The practice shows that the system as a nondestructive evaluation method can be widely used in the reliability evaluation of layered structures.