The terahertz time domain spectroscopy (THz-TDS) is used to quantitatively analyze the benzoic acid content in arrowroot, and the arrowroot is with a plastic bag. The original data are subjected to the pretreatments of multiple scattering correction, baseline correction, first derivative, second derivative, and so on. The partial least square (PLS) method is used to establish a prediction model for the detection of benzoic acid content in arrowroot. The detection results show that model-determining factor for samples without plastic bags is 0.975 and the predicted root mean square error (RMSEP) is 1.126%. In contrast, that for samples with plastic bags is 0.976 and the RMSEP is 1.356%.