Terahertz time-domain spectroscopy (THz-TDS) was utilized to investigate the solid-state reaction between L(+)-Tartaric acid and sodium hydrogen carbonate. Solid sodium hydrogen L(+)-tartrate monohydrate was synthesized efficiently by mechanical grinding, which is particularly sustainable and environmentally benign. Distinct THz absorptions were observed for pure reactants and the proposed product. The reaction process could be clearly visualized by THz spectral patterns of the reaction mixtures at different grinding time. The observed results were further confirmed by synchrotron radiation X-ray powder diffraction (SRXRPD) and Fourier transform infrared (FT-IR) spectroscopy. The study demonstrates that THz-TDS is an effective novel tool to monitor solid-state reactions in pharmaceutical industry.