To overcome the limitations of conventional methods for detecting azo synthetic dyes in tea, including complicated sample pretreatment, limited sensitivity, and poor suitability for rapid on-site analysis, a microfluidic tapered-fiber surface-enhanced Raman scattering (SERS) sensor based on Ag nanocubes-thiourea composite substrate (AgNCs@CH4N2S) was developed in this study. Silver nanocubes (Ag nanocubes, AgNCs) were prepared by a polyol reduction method and further functionalized with thiourea. This surface modification effectively reduced the negative surface charge of the SERS substrate and strengthened the electrostatic adsorption between the substrate and negatively charged azo dye molecules, thereby significantly improving Raman signal enhancement. The experimental results showed that the microfluidic platform achieved a limit of detection (LOD) as low as 10-9 mol/L for Rhodamine 6G (R6G), with an analytical enhancement factor (AEF) of 1.81×107, and enabled the simultaneous discrimination of multiple pollutant components. The LOD for four azo dyes, namely Sunset Yellow, Tartrazine, Carmine, and Amaranth, were all determined to be 1 mg/kg. The microfluidic SERS sensor developed in this work offers high sensitivity, rapid response, good integration, and suitability for on-site trace detection, providing a new approach for the rapid screening and accurate quantification of prohibited azo synthetic dyes in tea.