Terahertz time- domain spectroscopy provides us a powerful tool to carry out a fast and accurate measurement of optical constants of materials in terahertz range. The thickness determination of sample has a large influence on extracting the refractive index of materials. At the same time, the thickness of sample is generally difficult to be accurately measured. In order to avoid the influence from the measurement error of sample thickness on the determined refractive index, it is useful to develop a method which simultaneously determines the thickness and refractive index of sample. Due to the roundtrip reflected signal is weaker, the method presented by Duvillaret from the aspects of calculated frequency range and iterative algorithm is improved, making the calculated results more accurate and the operation more convenient. The thickness and refractive index of polyethylene and silicon wafer are determined as an example so that to verify the effectiveness of this method.