Amaranth is a synthetic food additive. It is often added to soda, hawthorn and candy, etc. However, Amaranth is made from the coal tar isolated by the benzene dye as raw material, and excessive consumption will lead to genetic mutations and even cancer, which is harmful to health. Therefore, the detection of Amaranth is important and necessary. Surface-enhanced Raman spectroscopy has the advantages of simple pre-processing, rapid analysis and high accuracy for samples, which has been increasingly shown great potential in the fields of chemistry, biology and medicine. At present, the theoretical and experimental research on Amaranth with Raman spectroscopy technology has not been reported. The study of Raman spectroscopy and the theoretical mechanism of surface enhancement can provide a reliable scientific basis for the detection and identification of Amaranth in food. Therefore, the study on the surface-enhanced Raman mechanism of Amaranth and compare it with the experimental results using the density functional theory is necessary. It has a good prediction and guiding significance for the detection of Amaranth in food. On the one hand, Amaranth was detected by using micro-confocal Raman spectrometer. On the other hand, the structure of Amaranth was built, and Density Functional Theory was used to optimize the molecular structure and calculate the front-line orbit, electrostatic potential, polarization and natural bond orbital analysis. And the azo group (-N15N16-) is the best position for Amaranth to coordinate with Ag atom. The optimum structure and surface-enhanced Raman spectra of Amaranth-Ag1, Amaranth-Ag3 complexes molecules were calculated by using B3LYP function with 6-31++G (d, p) (C, H, O, N, S, Na) base set and Sdd (Ag) base set, receptively. The results showed that the experimental results at 1 228, 1 329, 1 467 and 1 529 cm-1 were in good agreement with the theoretical results and the Raman activity of Amaranth molecule was obvious. Raman enhancement effect of Amaranth-Ag complex was significant and the Raman enhancement effect increases with the number of Ag atoms. It can be found that not only the numbers of Raman peaks increased, but also the corresponding intensity increased. Furthermore, the vibrational mode was assigned and the characteristic peaks for detection and inspection of Amaranth can be found. The study provides an experimental reference and theoretical basis for the detection of Amaranth by using surface-enhanced Raman spectroscopy.