The chiral isomers D- and DL-tartaric acid are measured by terahertz time-domain spectroscopy (THz-TDS), and characteristic absorption peaks of two isomers in the range of THz are obtained. The differences between them are obvious. The single molecular and unit cell of two isomers of tartaric acid are calculated by density functional theory in quantum chemistry software. The absorption peaks of two isomers of tartaric acid are obtained and identified. The results of the experiment are basically consistent with the calculated results at absorption peak. Through the identification of the absorption peak, the formation mechanism of the absorption peaks of such isomers in the range of THz is deeply analyzed. The results show that THz-TDS technology can identify small differences between tartaric acid chiral isomer and its racemate.