With the continuous progress of industrialization, heavy metal pollution in water is becoming more and more serious. Straw has many advantages such as wide availibility, low cost and easy production, which makes it an ideal biological adsorbent to deal with heavy metal pollution.In order to study the application prospect of terahertz wave technology in the detection of heavy metal pollution in water body, original straw and alkaline straw were selected to absorb heavy metal ions. Terahertz time-domain spectroscopy (THz-TDS) was utilized at a room temperature under nitrogen atmosphere to conduct spectral measurement for original straw samples, alkaline straw samples, samples of original straw with heavy metal ions absorbed, samples of alkaline straw with heavy metal ions absorbed, and samples of desorbed alkaline straw. The absorption coefficient of the alkali-treated straw under 1.75~1.85 THz hybrid absorption peak band was significantly decreased, accompanied by an acromion backward shifting phenomenon of the absorption band. After absorbing heavy metal ions, the intrinsic 1.75~1.85 THz hybrid absorption band was disappeared in original straw samples, but formed a hybrid absorption band in the range from 1.8 to 2.05 THz. Alkaline straw in 1.7~2.05 THz presented a number of complex acromino-absorption, while heavy metal ions absorbed alkaline straw showed a smooth and strong absorption band in 1.7~2.05 THz. In addition, the absorption spectrum of the hydrolyzed heavy metal ion absorbed alkaline straw was nearly parallel. Though integrating the chemical reaction process of the samples, absorption spectrum was analyzed, and the information of molecular vibration and functional group of different components were gained. Furthermore, the conclusion that complex adsorption and electrostatic adsorption are the two methods for alkaline straw to absorb heavy metal can be drawn. It proved the reliability and good application prospect of the terahertz time-domain spectroscopy on detecting heavy metal pollution in water.