Interval selection methods combined with terahertz time-domain spectroscopy (THz-TDS) technique were used to perform quantitative analysis of component concentrations in multicomponent mixtures. The THz spectra of 100 quaternary pharmaceutical mixtures composed of lactose monohydrate, acetaminophen, microcrystalline cellulose and soluble starch were measured using THz-TDS system. Four spectral interval selection methods, including iPLS, mwPLS, siPLS and biPLS, were employed to select spectral intervals of THz absorbance spectra of multicomponent mixtures and correlate THz absorbance spectra with the concentrations of lactose monohydrate. The mwPLS method yielded the most accurate result as compared with the other three interval selection methods and full-spectrum PLS. The optimal mwPLS model was obtained with lower root mean square error of cross-validation (RMSECV) of 0.980 3, lower root mean square error of prediction (RMSEP) of 1.114 1, higher correlation coefficient for calibration (RC) of 0.996 0, and higher correlation coefficient for prediction (RP) of 0.995 1. Experimental results demonstrate that spectral interval selection combined with THz-TDS could be successfully applied as an accurate and rapid method to determine component concentrations in multicomponent mixtures.