Abstract: Aiming at the difficulty in extracting the electromagnetic parameters of highly absorbing samples by using the transmission measurement system, the time-domain spectrum of polar and non-polar liquids were measured by the reflection time-domain spectroscopy system. The propagation characteristics of terahertz wave in the sample were modeled by finite integration time domain algorithm, and the reflection spectrum was reconstructed by using the Gaussian mixture model to restore the original terahertz time-domain signal from the noise, which solved the false fluctuations in electromagnetic parameters caused by the interference of signal in reflection system. The experimental results show that compared with the results calculated by using the original signal, the refractive index and extinction coefficient of water and liquid paraffin in the 0.4 THz~2 THz band calculated by using the restored signal, effectively eliminate the false fluctuations, and the detrended fluctuation analysis (DFA) scale index is increased by 7%, 3%, 29%, and 31%, respectively, which verifies the validity and correctness of the method.