The quantitative analysis for plasticizer based on terahertz (THz) time-domain spectroscopy and chemometrics is studied. A novel method for measuring the optical constants of thin liquid samples using THz transmission spectrum is presented. Taking a typical plasticizer, dibutyl phahalate (DBP) as an example, we use the method to determine the refractive index and absorption coefficient of DBP-ethanol and DBP-ethanol-water solutions with different DBP concentrations in THz regime, where a 200 μm path length cuvette is adopted as the sample holder. The results show that the optical constants of DBP-ethanol and DBP-ethanol-water solutions in the terahertz band (0.2-1.1 THz) change regularly corresponding to the DBP content. According to the average refractive index and absorption area the prediction model of DBP concentration in unknown solutions is constructed. The results demonstrate that the THz spectroscopy is a potential method for rapid identification of trace plasticizer in wine or other soft drinks.