Broadband transmission terahertz time-domain spectroscopy plays a significant role in research fields such as material carrier dynamics and fingerprint spectrum identification. Due to the support required for low-dimensional materials, the transmission characteristics of substrates are crucial in the study of the interaction between terahertz waves and low-dimensional materials. To investigate the optical properties of different substrate materials in the terahertz range, the terahertz transmission signals of three common substrate materials are tested at first. The refractive index and absorption coefficient of each material are then calculated within the effective spectral range. The results show that CVD diamond maintains small-dispersion refractive index and absorption coefficients (less than 10 cm^-1) in the frequency range of 1 THz～18 THz, making it suitable for use in broadband terahertz time-domain systems. While, fused quartz and magnesium oxide crystals exhibit significant dispersion in the broadband terahertz range, leading to a broadening of the transmitted time-domain signals, and they are only suitable for terahertz time-domain spectroscopy system studies in the low-frequency range (less than 3THz). Thus, the results provide an important reference for research based on broad-spectrum terahertz time-domain spectroscopy.