The Terahertz spectra of sweet potato starch, potassium alum and their compounds are measured in 0.3~2.0 THz using Terahertz time - domain spectroscopy ( THz-TDS) system. The refractive index and absorption coefficient spectrum are calculated. Experimental results show that potassium alum has distinct absorption features in the THz region, which can be used as fingerprint for the material identification. in addition, The Terahertz spectra of sweet potato starch doped with different percentages potassium alum are measured. The refractive index and absorption coefficient spectrum which are calculated indicate that the higher proportion of Potassium alum is, the larger the absorption coefficient in the compounds is. So is the refractive index. The results indicate that THz-TDS can be used to qualitative and quantitative detection ofpotassium alum in sweet potato starch.