Quantitative analysis of mixture samples’ components based on terahertz absorption spectra is one of the most important applications of THz technology. As is known to all, when the THz wave irradiates into samples, it will be scattered by the sample particles in the microscale, which is known as scattering. So THz absorption spectra of the experimental obtained samples are basically composed of two parts, samples’ intrinsic absorption of THz wave and the scattering. Especially when particles’ grain size of the samples is near or comparable to the THz wavelength, the scattering is so significant that it cannot be ignored. However, the scattering was always not taken into account while only the Lambert-Beer law was used in THz quantitative analysis, in which the absorption of light was described as linearly related with the material’s concentration. As a result, the accuracy was restricted. In this paper, the scattering in THz band was analyzed and absorption spectrum model of mixed sample’s THz was proposed. A series of quantitative analysis experiments proved the validity of this model. Through the comparision with the traditional method, the accuracy of the quantitative analysis was improved and the errors were basically below 3%. This research proved that scattering is important to improve the accuracy of THz quantitative analysis.