Terahertz time-domain spectroscopy (THz-TDS) is a significant technique for characterizing materials as it allows fast and broadband measurement of optical constants in the THz regime. The measurement precision of the constants is highly influenced by the complicated measurement procedure and data processing. With particular emphasis on normal reflection measurement, the sources of error including amplitude errors and positioning error and sample misalignment in determining the dielectric properties of materials were analyzed. The uncertainty that characterizes the influence on the optical constants for each source was formulated independently. Several simulations were carried out to illustrate the relations between the error sources and the uncertainty of the optical constants.