Based on the Attenuated Total internal Reflectance Fourier Transform Infrared spectroscopy (ATR-FTIR) to study the main differencesof chemical constituents in water extraction of fresh Radix Rehmanniae (RR), a decoction of crude RR and processed RR. In addition, ion chromatography was used to analyze the different components quantitatively. This study aims to reveal the whole composition and variation of fresh RR and its processed products. In ATR-FTIR, there were differences among the three RR in the region of saccharides. The characteristic peaks of fresh RR were located at 1 140, 1 047 and 1 000 cm^-1, raw RR were 1 140 and 1 045 cm^-1, and processed RR were 1 142 and 1 029 cm^-1. Moreover, the peak shapes of the three kinds of RR were quite different. Meanwhile, the second derivative infrared spectra (SDIR) showed that the characteristic peaks of saccharides in the range of 1 200~600 cm^-1 changed most significantly during the processing of RR. The positions of characteristic peaks of three kinds of RR extracts were similar, but the relative peak intensities were different, and they changed regularly with the processing process. It can be inferred that during the processing of fresh RR, the polysaccharides in fresh RR hydrolyzed into oligosaccharides or monosaccharides. Furtherly, 8 monosaccharides and oligosaccharides were analysed quantitatively using ion chromatography with good linear relationship (R²≥0.999 0). The results indicated that the contents of glucose, melibiose, galactose, mannose, manninotriose, stachyose and acteoside were different in the extracts of the fresh RR, crude RR and processed RR (p<0.05). The content of stachyose and fructose was the highest in fresh RR and raw RR, respectively. The other 6 saccharides trend to increase in processing, presumingthat the stachyose in fresh RR might havea multi-channel hydrolytic process. This study explored the regulation of the whole chemical components of fresh RR in the processing process and provided data support for laboratory investigation and the clinical application of fresh RR. Moreover, these data provided a reference for quality control of fresh Chinese herbal medicine and gave new ideas for traditional Chinese medicine processing research.