Residues of harmful dyes such as Auramine O (AO) in herb and food products threaten the health of people. So, fast and sensitive detection techniques of the residues are needed. As a powerful tool for substance detection, terahertz (THz) spectroscopy was used for the quantitative determination of AO by combining with an improved partial least-squares regression (PLSR) model in this paper. Absorbance of herbal samples with different concentrations was obtained by THz-TDS in the band between 0.2THz and 1.6THz. We applied two-dimensional correlation spectroscopy (2DCOS) to improve the PLSR model. This method highlighted the spectral differences of different concentrations, provided a clear criterion of the input interval selection, and improved the accuracy of detection result. The experimental result indicated that the combination of the THz spectroscopy and 2DCOS-PLSR is an excellent quantitative analysis method. Display omitted •Terahertz spectroscopy was applied for quantitative determination of harmful dyes in herbs.•2DCOS-SPLSR (partial least squares regression based on two-dimensional correlation spectroscopy) was presented for model optimization.•2DCOS-SPLSR highlighted the spectral differences of different concentrations, provided a clear criterion of the input interval selection, and improved the accuracy.•Terahertz spectroscopy combined with 2DCOS-SPLSR is a fast, accurate and sensitive method of quantitative determination.