In this article, we introduce an analytical method for measuring the quantitative ratio of components in a mixture, using terahertz time-domain spectroscopy (THz-TDS) in the ambient atmosphere. A reconstructed terahertz time-domain signal was used to filter out oscillations caused by water vapor. The reconstructed signal was found to be very similar to a reference sample signal that was measured after purging with dry air or nitrogen. Since the amplitude of the reconstructed signal is not the same as the reference sample signal, the sample absorption coefficient cannot be calculated. In this article, the concentrations of monosaccharide mixtures were measured, based on their refractive index, using THz-TDS in the ambient atmosphere. The results confirmed the feasibility of the concentration analysis method, and we believe this method will benefit analytical chemistry or nondestructive characterization industries in real-world applications.