This study proposes a method based on material dispersion models to computationally simulate terahertz (THz) time-domain spectroscopy signals. The proposed method can accurately extract the refractive indices and extinction coefficients of optically thin samples and high-absorption materials in the THz band. This method was successfully used to extract the optical constants of a 470-μm-thick monocrystalline silicon sample and eliminate all errors associated with the Fabry–Pérot oscillation. When used to extract the optical constants of a 16.29-mm-thick polycarbonate sample, our method succeeded in minimizing errors caused by the low signal-to-noise ratio in the extracted optical constants.