The continuous dehydration processes of adipose, muscle and skin from pork were investigated by transmitted terahertz time-domain spectroscopy (THz-TDS). The changes of terahertz spectroscopy during the dehydration have been observed, and the difference of absorption coefficient and refractive index at the beginning and end of the dehydration among three biological tissue samples were compared. The results show that terahertz spectroscopy is sensitive to the change of water content in the fresh tissues, and water content is the primary factor to influence the absorption coefficient of fresh tissues. Moreover, there are significant differences in the absorption of the terahertz spectroscopy for different tissues, indicates that the transmitted THz-TDS enable to distinguish different biological tissues.