Terahertz (1THz = 10^12 Hz) time-domain spectroscopy as a new type of spectrum detection technology,is widely used to study a variety of materials in the far-infrared spectral range of the optical properties.In most cases,it needs to know the thickness of the sample in order to accurately extract the optical parameters of samples in THz transmission spectral analysis.However,some of the thickness of the sample can not be measured with accuracy even can not be measured directly.According to error theory,we propose a method to determine the thickness of the sample accurately,while sample′s THz band optical parameters of method can be extracted.It can be widely used in such samples′ THz spectrum analysis.