Based on the optical fiber evanescent-wave excitation and surface-enhanced Raman scattering (SERS), we have carried out experimental studies on SERS characteristics of the 2×2 fused fiber coupler coated with Ag colloid particles. By stretching a pair of fused single-mode fibers with coating Ag colloid on its surface, the formed component can work as the surface-active substrate to perform Raman enhancement. With increase of the taper length, fiber core diameters decrease and less energy are bounded in the fiber. This enables stronger evanescent-wave to leak outward and become a Raman exciting source. In the experiments, the Raman spectra of R6G solutions has been detected at the coupling zone. The minimum detected concentration is 10^-8 mol/L.