In recent years, food safety problems caused by illegal additions in infant foods have received widespread attention. Surface-enhanced Raman scattering (SERS) technique is used to rapidly and non-destructively detect the banned RhB that is usually added in food. In this study, we have prepared g-C₃N₄/Ag composites via a simple method successfully, their morphology and structure were characterized by transmission electron microscope (TEM), ultraviolet-visible (UV-Vis), X-ray diffraction (XRD), fluorescence spectrophotometer and confocal micro-Raman spectrometer (Raman). The g-C₃N₄ nanosheet possesses good adsorption performance due to its highly delocalized π-conjugated system, which acts as a carrier for Ag nanoparticles. Therefore, Ag nanoparticles are more uniformly and stably distributed on the surface of g-C₃N₄ nanosheets to form g-C₃N₄/Ag nanocomposite, which can be used for rapid adsorption and trace detection of RhB. In the experiment, the pH of the test and the absorbed time between the substrate and RhB were optimized. The influence of pH on the SPR of the substrate and the SERS intensity of the probe molecule were investigated in detail. As g-C₃N₄/Ag nanocomposite shows a significant higher absorption in the visible region around 500 nm than Ag nanoparticles, g-C₃N₄/Ag nanocomposite is more favorable for SPR absorption. A wide SPR absorption range is achieved due to the synergy between g-C₃N₄ and Ag nanoparticles, providing an improved SERS enhancement performance. Under the optimal experimental conditions by using RhB as probe molecule, an enhancement factor of 7.6×10 ⁵ is achieved. Due to the electrostatic interaction and π-π interaction between the substrate and the probe molecules, the substrate can enrich in a large amount of cationic dyes, offering a detection of RhB. The g-C₃N₄/Ag SERS substrate can be used to detect RhB with a linear relationship from 1.0×10 ^–9 to 1.0×10 ^–6 mol/L and a detection limit as low as 0.39 nmol/L. In addition, the g-C₃N₄/Ag nanocomposite SERS substrate can also detect trace amounts of RhB molecules in the commercially available rainbow lollipops with a high sensitivity, and the recovery were 93.6%~95.04%. In summary, the g-C₃N₄/Ag nanocomposite is not only a SERS substrate with high sensitivity, uniformity and stability, but also can be used as a rapid trace detection method of Rhodamine B in real food and environment.